Why is this error being shown?
'Could not submit transaction to Governor contract. . Internal JSON-RPC error.'
because there is an error in your contract
This usually means that according to the current blockchain state you can't create a Governor proposal (note this is different from a "Defender Admin proposal", naming is hard!).
In my experience, most of the times I've seen this error pop-up in Defender is due to the sending account not fulfilling the criteria to be able to create these proposals.
An example of this situation is when you don't hold the minimum amount of the Governor's ERC20 token that would entitle you to propose.