"If one has trouble swallowing tablets or capsules, you could try taking medicine in liquid form."
The answer key states there is one error, and that is apparently "one has". What exactly is the reasoning for this being wrong? It seems fine to me.
The problem is not inherent in "one has."
The problem is that the sentence uses "one" in the first clause and then switches to "you" later on.
It should be consistent: either one has... one could or (and this is more common IMO) you have... you could.
Two ways to change it: “If one,,, they could”. Or “If you… you should”. If one, if you: Both correct. They could, you should, both correct. But you need a matching pair of words.