P. 24 of the installation manual has trouble-shooting suggestions that mention the shear pin but also advises the shear pin is in a flexible coupling that must be properly engaged to the vertical driveshaft. It also mentions checking the propeller fasteners and the key in the propshaft keyway.
(Click on Files at the right & download the manual if you don't have it with you.
https://side-power.com/produkt/38149/se40-tunnel-thruster-12v/
"If the electromotor runs but there is no thrust, check:
-If the flexible coupling between the motor and driveshaft is
not fitted correct inside the boat. *
-Are the propellers in the tunnel fastened correctly on the
prop-shaft (key/drive pin present) **
-With the motor removed, turn the driveshaft from inside
the boat to feel if the gears are engaging and turning the
prop-shaft. ***
Solutions :
*Check the flexible coupling/shear pin and the motor installation to ensure correct connection of the flexible coupling before re-fitting the electromotor.
**Re-fasten or replace the propeller and/or key/drive pin.
***In case of a failure inside the gearhouse, we advice (sic) to get a replacement gear-house
instead of attempting to repear (sic)the internal gear and bearing system. "
Hope it is something simple.