The Simple Start version is capable of converting estimates to invoices, Clayco.
Let's check if a full cache causes this error.
A cache store your browsing data to reload the web pages you've visited faster. If it's full, it can cause slowness or unusual behavior in the software.
Please open a private browser, log in to QuickBooks, and convert your estimate. It it works fine, you can go back to your regular browser and clear the cache. However, if the same thing happens, you can try other browsers to isolate the case.
Please let us know how it goes so we can help you further.