Unable to Update Mono to Version 5.18 on Debian 9.8

I'm unable to update Mono from version 4.6.2 to 5.18.0 on Debian 9.8. I followed the instructions on the download page on Mono's website but with no luck so far.

{bash}
root@server ~ # mono --version
Mono JIT compiler version 4.6.2 (Debian 4.6.2.7+dfsg-1)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
        TLS:           __thread
        SIGSEGV:       altstack
        Notifications: epoll
        Architecture:  amd64
        Disabled:      none
        Misc:          softdebug
        LLVM:          supported, not enabled.
        GC:            sgen

root@server ~ # cat /etc/apt/sources.list.d/mono-official-stable.list
deb https://download.mono-project.com/repo/debian  stable-stretch main

root@server ~ # apt update
Hit:1 http://security.debian.org  stable/updates InRelease
Hit:2 http://security.debian.org  testing/updates InRelease
Ign:3 http://deb.debian.org/debian  stable InRelease
Hit:4 http://ftp.debian.org/debian  stretch-backports InRelease
Hit:5 http://deb.debian.org/debian  stable-updates InRelease
Ign:6 http://ftp.de.debian.org/debian  stable InRelease
Hit:7 http://deb.debian.org/debian-security  stable/updates InRelease
Hit:8 http://deb.debian.org/debian  stable Release
Hit:9 http://ftp.de.debian.org/debian  testing InRelease
Hit:10 http://ftp.de.debian.org/debian  stable Release
Hit:11 https://download.mono-project.com/repo/debian  stable-stretch InRelease
Hit:12 https://packages.sury.org/php  stretch InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
2 packages can be upgraded. Run 'apt list --upgradable' to see them.

root@server ~ # apt list --upgradable
Listing... Done
libgd3/testing 2.2.5-5.1 amd64 [upgradable from: 2.2.5-5+0~20190119054529.2+stretch~1.gbp911a4a]
libsodium23/testing 1.0.17-1 amd64 [upgradable from: 1.0.17-0.1+0~20190131101845.5+stretch~1.gbp8cf594]

--- **Update 1** Added sudo apt install mono-devel mono-complete output.

{bash}
root@server ~ # sudo apt install mono-devel mono-complete
Reading package lists... Done
Building dependency tree
Reading state information... Done
mono-complete is already the newest version (4.6.2.7+dfsg-1).
mono-devel is already the newest version (4.6.2.7+dfsg-1).
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.

--- **Update 2** /var/lib/apt/lists/ only has partial directory which is empty.

{bash}
root@server /var/lib/apt/lists # ls -al
total 28
drwxr-xr-x 3 root root 12288 Feb 23 07:53 .
drwxr-xr-x 5 root root  4096 Feb 22 20:38 ..
drwx------ 2 _apt root 12288 Feb 22 20:38 partial

root@server /var/lib/apt/lists # cd partial/

root@server /var/lib/apt/lists/partial # ls -al
total 24
drwx------ 2 _apt root 12288 Feb 22 20:38 .
drwxr-xr-x 3 root root 12288 Feb 23 07:53 ..

Error on running apt full-upgrade.

{bash}
root@server ~ # apt full-upgrade
Reading package lists... Done
E: The value 'stable' is invalid for APT::Default-Release as such a release is not available in the sources

I'm getting the same error after running aptitude as shown in the screenshot. --- **Update 3** Added /etc/apt/apt.conf.d/99server output.

{bash}
root@server /etc/apt/apt.conf.d # cat 99server
Acquire::PDiffs "false";
Acquire::Languages "en";
APT::Default-Release "stable";

--- **Update 4** When I try to force install Mono 5.18.0 using sudo apt-get install mono-devel/stable-stretch I get the errors shown below.

{bash}
root@server ~ # sudo apt-get install mono-devel/stable-stretch
Reading package lists... Done
Building dependency tree
Reading state information... Done
Selected version '5.18.0.240-0xamarin1+debian9b1' (Xamarin-Stable-Stretch:9.0/stable-stretch [all]) for 'mono-devel'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libmono-system-web-services4.0-cil : Depends: libmono-system-design4.0-cil (>= 1.0) but it is not going to be installed
 mono-devel : Depends: libmono-i18n4.0-cil (>= 4.0.0~alpha1) but it is not going to be installed
              Depends: libmono-system-design4.0-cil (>= 1.0) but it is not going to be installed
              Depends: libmono-system-drawing-design4.0-cil (>= 1.0) but it is not going to be installed
              Depends: libmono-system-messaging4.0-cil (>= 2.10.1) but it is not going to be installed
              Depends: libmono-system-runtime4.0-cil (>= 2.10.1) but it is not going to be installed
              Depends: libmono-system-servicemodel-activation4.0-cil (>= 1.0) but it is not going to be installed
              Depends: libmono-system-servicemodel-web4.0-cil (>= 3.2.1) but it is not going to be installed
              Depends: libmono-system-servicemodel4.0a-cil (>= 3.2.3) but it is not going to be installed
              Depends: libmono-system-web-extensions4.0-cil (>= 2.10.3) but it is not going to be installed
              Depends: libmono-system-web-webpages-razor2.0-cil (>= 1.0) but it is not going to be installed
              Depends: libmono-system-web-webpages2.0-cil (>= 1.0) but it is not going to be installed
              Depends: libmono-system-windows-forms4.0-cil (>= 1.0) but it is not going to be installed
              Depends: libmono-cil-dev (= 5.18.0.240-0xamarin1+debian9b1) but it is not going to be installed
              Recommends: msbuild but it is not going to be installed
 monodoc-manual : Depends: monodoc-browser but it is not going to be installed or
                           monodoc-http but it is not going to be installed or
                           monodoc-viewer
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.