The repository 'https://cloud.r-project.org/bin/linux/ubuntu bionic Release' does not have a Release file

My computer has Linux Mint 19.1 Tessa with R version 3.6.3. I am trying to do sudo apt-get update and I am having this error:

Err:21 https://cloud.r-project.org/bin/linux/ubuntu  bionic Release             
  404  Not Found [IP: 2600:9000:2045:e400:6:c2d3:f940:93a1 443]
E: The repository 'https://cloud.r-project.org/bin/linux/ubuntu  bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.

Also, I would like to install appropriate drivers but I am also getting this error:

E:The repository 'https://cloud.r-project.org/bin/linux/ubuntu  bionic Release' does not have a Release file.

` Could someone help me? I remember that I had a problem upgrading R, but I am not sure what happened with Release. R now is working normally I think.