Windows partition not seen

I have a pc which was running Windows 10 and I wanted to create another partition and dual boot Pop-OS. I followed instructions to create a bootable USB drive and use Gparted to make a new partition. Installed Pop-OS and everything seemed good. Linux now running on my machine. However, I noticed that there was no option after POST to select which os I wanted to run. I found a command (11 year old post) to hold SHIFT down while booting and it showed me the blue Grub screen with operating systems on it and only the Pop GNU/Linux option was listed. Another post (dated 2018) suggested running 'gnome-disks' to see what is there and it showed the two partitions for Windows and the two for Linux. Only the Linux was mounted, so I mounted all of them and rebooted, but to no avail. I ran 'ntfsfix /dev/sda2' where sda2 is my ntfs partition for Windows, and got this error: 'Refusing to operate on read-write mounted device /dev/sda2.' I am still learning the system, but have learned so much already because of this problem. I need some help please.