Hi,
Normally, for PHP client library, if the error being generated states that it cannot find a certain class, then it means that the dependencies are not fully installed/downloaded. I have also encountered this issue before for the OAuth2TokenBuilder when I installed the latest PHP client library locally. What I did to resolve the issue was to install again the client library using composer as you can see on this
guide just to make sure that all dependencies are downloaded.
On your end, if you haven't, can you try to install the client library again based on that guide and let me know if the issue still persists?
Thanks,
Vincent
AdWords API Team