Hey there.
What do the logs say about the import of coverage field?
LCOV is not a supported file format for importing test coverage on C/C++ code. Those formats are specified in the documentation you already linked to.
There are also extensive examples in the docs:
In the same GitHub organization, you will also find example repositories that provide guidance on how to add coverage to an already-configured project. These examples do not explicitly describe every possible combination of tooling and platform but do cover the most significant variants. You may need to adapt them slightly: