Downloaded the update to the Simple Start $25/mo plan & now when I try to submit/save/print/do anything it says "Sorry there's a problem with your subscription."

Welcome to the Community, @ajtarbart42. I'm here to help you get back to business.

I'd like to start with some troubleshooting to see if a browser-related issue.

The best way to handle an error message is to clear the cache and cookies of the browser you're using for your QuickBooks Online. These files can become outdated, causing issues like the one you're having when creating transactions and receiving an error message. Here's a link to help you with the process: Clear Web Browser History. 

Let me know if this works for you! I'll be on standby waiting for your response.