Can you get sacked if you can't do something?

[Sarah]

Eight weeks ago I was employed as your average administrative assistant. I did a HND in administration at college and as far as I am aware I should be doing word, excel, access, filing, mail etc etc.

The company I was employed by was two years old with NO administration in place. So I had to fix it as best I could. Now my boss says I have to build a second and third website for him. And having NO website building experience that was a challenge aside from the fact it doesn't come under 'administrative assistant' at all.

Then he thinks I should be making sales calls. I am not a salesman but this I kind of understand and will eventually learn.

Then he thinks I should market his company with SEO, registering in yell etc. Facebook, twitter and the like. Not really 'admin' but its understandable to a degree.

And now he has hit me with 'I want to eventually get rid of my accountant and you can do all that'. I was gobsmacked. I am not an accountant, I don't have a clue about accountancy and unless I went to do a degree on it then I think I will do it incorrectly and it will be relatively impossible.

I do ALL of this for minimum wage. Basically I run the company.
This is my first job. I have bee there for 8 weeks. I don't understand accountancy and I will never be able to do it and now I am afraid that he can sack me because I can't do it. Is this allowed?