I don’t know how your product works but I assume the part (mounting plate) that is stuck on was supposed to go on in the other direction so when you mounted the motor attachment piece the motor would be facing in the correct direction.
Did the “kit” come with a manual that you could post here?
Then perhaps you could provide some detail about why and how mounted the Flipsky kit to your board.
Show some other photos of the full bottom of the board etc.
The SUP would have some sort of finbox to mount the original keg/fin in. Did you stick a component of the Flipsky kit on top of that and it can’t be removed?