Hi,
there is nothing to fix, this happens because you chosen to create your own Google App but you did it incorrectly.
You chosen the wrong type, it must be not a 'web application' but of type 'Other' as reported in the docs.
Duplicated here: https://storejextensions.org/forum/view/10-jsitemap/3402-solved-google-app-client-id-credentials.html

NOTICE: you don't need to create your own Google Application, JSitemap already works with the default built in Google App so you can make your life easier and leave all settings empty as by default to use the built in Google app