I just opened a project made in unity version 2018.1.1f into Unity version 2020.1.1f I know opening the older projects is not supported by Unity but it opened fine.
What I tried:
- checking the script
onetizationPlatform.csto ensure nothing is wrong in it
- checked that there's no two files with same name
- doing point 2 with
unityengine.advertisement.editorwhich caused another error so I had to restore it back