Pretty good one to test this. If only I could think of something I can't do... only joking. That's a pretty long list.
Train driving, bomb disposal, technical drawing, hairdressing, cheese making, non-invasive surgery, invasive surgery, tree surgery, animal husbandry - I could go on. There's a lot I can't do.
Some of the things I can't do are quite dangerous so that's not a path I should be going down. Endangering students and myself would surely have quite expensive insurance implications.
In the end I settled on teaching a musical instrument. Can't play one. Tried once or twice, got so far (not very far) and then gave up when I found it got difficult. I can't read music either so that must make twice what I can't do in one teaching.
As I'd never even tried learning the piano that seemed a good one to teach. I had a look at some adverts for piano teachers in my area, copied them and knocked a couple of quid off the fee before posting.
I didn't have long to wait before someone contacted me. They asked if I taught grade five. I didn't know what they were talking about so said yes. We booked a lesson for the following day.
Nice people. Lady and her two children. Obviously a bit disappointed that I don't have a piano and that they'd driven 25 minutes across town for a piano lesson but they only shouted a bit, cried a bit and refused to pay. It didn't get violent or anything.
The lady did go on social media to tell people that I was a time-waster and a charlatan (I have never been in The Charlatans so she got that wrong) and warning everybody off signing up with me for a lesson.
And that meant my not doing and teaching career was over pretty much before it started. Shame.
Conclusion: Either I picked the wrong thing that I couldn't do to teach or maybe the world's just not ready for a teacher that can't do what they're teaching.