Hello Omri,
If you are using ManagedCustomerService and facing the USER_PERMISSION_DENIED error. Which means, you might be performing the actions to the account that you don't have access to. Please note that expanding invitation should be initiated via Manager account credentials and accepting/rejecting those invitations should be done with the credentials of that specific user. You could refer to the linking section of
this guide for more information.
If the problem persists, Please share the SOAP logs(
request and response) without redacting any information and the email of the user authenticating the call. To enable logging you could refer to the logging section of the README.md file in the client library of your interest. You could use reply privately to the author option while sharing the information requested.
Regards,
Sai Teja, AdWords API Team