I’m glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I’ll repost your solution in case you’d like to "Accept " the answer.
Issue: Unable to add paid parallel jobs to Azure DevOps after setting up billing. The user updated the billing to an Azure subscription, verified the corresponding resource group was created, but when trying to change the number of paid parallel jobs, the page throws a generic error message.
Solution: Resolved by @William Keen .
The issue was likely caused by background jobs running that needed to be completed before the billing functionality could be fully utilized. After waiting, the user was able to update the values. It is possible that the system required some time for processing background tasks or updates before allowing changes to the parallel jobs.