Cash vs Accrual Invoices + Invoice Operation Help

I am trying to test out some invoice operations using the execute endpoint, but I am running into a lot of 501 OperationNotAvailable errors.

Firstly, I am wondering if this specific error generally means that there is something wrong with the payload that we are sending in the request.

Secondly, I am wondering if certain invoice operations are not available depending on the accounting type of the invoice, ie cash vs accrual. For example, is running ProcessAccrualAdjustment on a cash invoice allowed and is running ProcessCashAdjustment on an accrual invoice allowed?