The access is denied because of the deny assignment with name – System deny assignment created by the managed application.

If you use Managed Application (usually third-party appliances) you can see this error. This is because usually a third-party vendor is responsible for it and is responsible for actions that you can perform. Sometimes there is a special group that can have permission to do some operations. Just go to the resource group, with this error and then to the IAM. Select Deny assignments:

And then select the entries, so you should be able to see Deny assignment excludes and you can try to add the user that needs permissions to that group.