If you're trying to creating a new Custom URL on the instance, it may fail to be created and will throw the below error:
"Unable to submit your new Custom URL request because another Custom URL request for your instance is still in progress."
This may happen if you previously created a custom URL but the job for provisioning it has not yet completed.
Custom URL jobs may take up to 6 hours to complete and polling for custom URL job completion occurs every 30 minutes.
You need to wait for the provisioning of the custom URL job that's in progress to complete before creating another custom URL.