No lines with coverage information in this diff.

[charismats2]

I removed some unit tests from the main branch locally and then I generate a coverage report with
dotnet test /p:CollectCoverage=true /p:CoverletOutputFormat=cobertura /p:CoverletOutput=./maincov.xml
after that I ran this command on windows cmd
diff-cover maincov.xml --html-report diff-coverage.html
but this message appeared
No lines with coverage information in this diff.

I checked the XML file generated by dotnet test and it detects not covered classes properly.
what is wrong in my process?

[Bachmann1234]

So I'm not 100% sure but it sounds like you were still on main. Diff cover will compare your current branch with main by default. So when it ran the diff it likely found no diff.

If you take your main branch with the tests then checkout a new branch and remove the tests from that branch.

Then try running diff cover from that new branch and see if they works better.