I had to upgrade my parent’s pc which was running a linux mint 17.3. It was no problem to upgrade 18, but then mintupdate app did the sam again and again, asked to install 18, then checked some things, all 'already the newest", done. I did this around 5 times, no option for 19 upgrade appeared, not even with apt dist-upgrade, which showed nothing to upgrade. For all packages apt writes nothing to upgrade. For apt install mintupdate it writes no newer relase, so ive purged it, then tried to reinstall, and writes mintupgrade not found, using the same command line that i used on 17 before the upgrade. Now how can i continue upgrading to newer mint?
Are you on 18.3? It will only allow upgrade from 18.3. I’m looking into it. There’s no direct path to 20 or 21 from 18, but from what I can see, you should be able to get to 19, then 20 once there, and on to 21 (like you’re attempting to do).
“[…] you can also upgrade 18.x to 19, which is supported until 2023. However, you need to first upgrade to 18.3 using the official instructions before upgrading to 19.”
https://blog.linuxmint.com/?p=3462