Ny way I can update macOS to Catalina after I used ignore command in terminal

Throw away your installation (saving any database files you need / export them) and then install the version from https://brew.sh

brew install mongodb
mongod --config /usr/local/etc/mongod.conf

You should be able to get v4.0.3 running without error and then compare if your distribution is compatible or needs adjustment if you can't use the mainline version for macOS.

Installing the program from the command line bypasses the gatekeeper checks and such and might be easier while the beta is new.

It is not your command line tools which are failing. Quite obvious, this is a compiler error and it seems like the python library was developed against a different set of headers - the compiler merely states a mismatch.

So gcc is working as expected - the python library‘s C extension is outdated. I fear there is little you can do aside from what you presumably already have done, namely opening a ticket in the MuJoCo-py project.

P.S. It would be nice if you added the information that you already opened a ticket in your question. Even if you did so after you asked here, you can always edit your question.