lets assume

You receive a job offer for a certain role. one that fits your experience greatly.
You join the job.

As soon as you join the job the boss decides that you should do a completely different role (one that they told you they would have another person to do this role) But you are not very good at this role as you don't have experience. You are interested in it but you don't know what you are doing. And nobody is showing you how either.

So boss gets upset you are not doing role properly and you start to worry they might get rid of you where does one stand? obviously in the contract there is the old ask you to do different tasks from time to time, line.