Hi,
The error message you are getting here is not really related to your use of the API but to the number series that is defined for Credit memos in Business Central. That no. series is not defined to allow for manual numbers.
So there is two options you can try. Either you can try to create the credit memo via the API but not giving a value to the number field. Then the number should be assigned from the defined number series.
If that is not working you will have to go into the number series in Business Central. Find the number series called "F-FAKTURA" and activate the field called "Manual no.s".
And the sales credit memo works a bit different from the sales invoice when it comes to quantity. In the sales credit memo you only have one quantity field. So you give that the quantity you want to return / refund on this particular credit memo.