update:
Ive managed to find my build.gradle in my project folder for unreal but any changes I make are overwritten on build. So it must be somewhere else I have to change it.
Ok ive found the billing area and it says I have 6.0.0, I should be up to date but its throwing errors still.
I am facing the same issue. I have ported a game from 5.3. The abb file generated with 5.3 had no errors, but with 5.4 I get the same error as you. The weird thing is that I have not changed anything related to the ads between the two versions.
I’d be interested to hear if anyone else if facing the same issue or has found a solution.