It seems Scala 3 generates coverage information for third party libraries with macro features and then report can't be generated because sources are not available for them.
Since for Scala 3 the coverage instrumentation is actual in the compiler (and I see you created an issue there already) I'm going to go ahead and close this.
Plugin Version: 2.0.0-RC2 Scala Version: 3.2.0-RC1
It seems Scala 3 generates coverage information for third party libraries with macro features and then report can't be generated because sources are not available for them.
Example project to reproduce the issue https://github.com/arixmkii/macro_cov_3.2.0-RC1
I will report it as well in dotty github to ask if this behavior is really desired.