Hi @TheDreamEXE ,
Please note that Unity IAP 1.20.0 and 1.21.0 don’t support Unity 5.3.x. So please use a previous version .
I imported a Unity IAP package in Unity 5.3.4 and didn’t get any error.
Could you please try it out in a new project?
Could you please send me a project which could reproduce this issue?
Once you import Unity IAP successfully, you shouldn’t have ‘UnityIAPInstaller.cs’ file any more.
And please confirm that you don’t have any compile error when importing Unity IAP. Otherwise, the installation process will be interrupted.