I can’t find the wheel nut anywhere! 
what are the next steps?

If you have the code card then you can order a new key.

If not, you can either pay a garage to break the 4 nuts off and fit a new set.  Or take a high quality picture of one of your nuts which can be used to decode it and aftermarket sellers make replacement keys on ebay.

My advice:

1 - buy four new normal nuts.  Make sure you get the correct shaped ones.

2 - go to a decent tyre fitting company.  They should have brass/deformable keys that they fit/bash onto the security wheel nuts, and then they remove them easily.  Put the new nuts on (yourself ideally, as the tyre-fitters will probably put them on too tightly)

Nobody steals wheels anymore, and if somebody really wants to steal your Fiesta wheels, they will regardless of whatever security nuts you have.

Don't know if you've had the car from new or acquired it recently.

The supplying dealer should have covered this at handover and given you the key code card.

It's normally in its dedicated slot in the foam insert which holds the tyre repair kit or tool kit in the boot. However, some dealers move them elsewhere. My usual dealer habitually puts them in the armrest cubby, and I've heard of others putting  them in the glovebox. So it could be worth having a look in the less obvious places.