Firstly, resistance relates to HIV and not the person. So an HIV negative person can't be drug resistant.
Drug resistance is only a risk if you become HIV positive but don't yet know this. This is especially likely with low adherence, even though the risk is still low.
Drug resistance affects PrEP in two different ways, both of which are very rare.
Starting PrEP without knowing that you are already HIV positive. This is why the HIV test before PrEP is essential. It is also why some people need to re-test after six weeks.
Becoming HIV positive during a break from PrEP and not re-testing before starting again.
Missing too many PrEP doses, so that drug levels are too low to prevent HIV infection.
In this case the drug resistance will stop PrEP from working.
Very few cases have been reported of PrEP not working because of drug-resistant HIV.