The issue is that the Google Play Billing Library introduced dependencies in 6.0.0 which can conflict with other dependencies in your project.
I compiled a list of solutions that solves most of these problems:
If this still doesn’t solve your problem, make a post with a list of dependencies and Unity editor version your project uses so we can reproduce the issue and help you solve it.
@jm6697 Could you confirm if the solutions above works for you? If not, let me know and I’ll test it with the dependencies you mentioned above.