Mounting a heavy (21 Kg) fan on a shed wall
(1) The fan as shown in the photo is upside down. Mounted in this position, the weight of the fan will pull the plate away from the beam. Turn it over so the weight will push the plate against the beam.

(2) The beam will hold the fan but the weight will apply a twisting force to the beam.

To overcome this, you should mount a board vertically from the beam to another anchor point, perhaps the steel beam directly above. Mount the fan on the vertical board, not directly on the beam.

Alternately you may be able to brace the fan mounting with a chain or other tension member between the end of the fan mount arm and the steel beam directly above.

Of course I'm not there so you'll have to identify and calculate the forces yourself.

(3) If you can, use through bolts with fender washers rather than large wood screws.