You want automated notifications of rate plan usage.
The latter is not supported. Re the error, I suggest trying to new UI, and checking browser network log, also Management Server logs and as a secondary workaround try and create the Rate Plan via Management API.
Thanks @dknezic , for your reply, but I have two questions.
What would be the most effective method to implement notifications that alert developers when they reach 50%, 80%, and 100% of their allocated rate plan limits?
Are there any APIs to retrieve the rateplan request quota and the user usage count so that I can subtract them from each other?