What is this Form over quota error?

That means that the form has reached the maximum number(limit) of submissions.

It is explained here: https://www.jotform.com/help/173-Understanding-Different-Form-Errors

6. Form over quota - As explained in the error message, the form has passed it's allocated quota and cannot be used at the moment. This error appears when you reached your submissions limit allocated for the month. The form will not be able to accept submissions any longer when you passed the submissions limit. It's either you wait until your submissions counter resets to zero which happens every first day of the following month or upgrade to any of the available paid plans to increase your submissions limits and enable all your forms right away. Every Jotform plan has its monthly submissions limit. See our pricing page for more details.

See the screenshot below to quickly see the differences of submissions limit depending on the pricing plan.