File a support Case on Google Admin
Sometimes the issue is at Google's level.
In this case here is how you can file a support ticket:
1 Go to : https://cloud.google.com/support-hub/
2 Click on "File a support case".
3 Click on "Cases: Create a case"
If the issue is related to an account not seen by us when updating the signature here is a text sample:
We are using Google's API to update our email signatures. We can't update the signature for email firstname.lastname@example.org