@Ben4954
Just be honest and upfront about it. In fact, if I was you, I would not only mention it to the guest before they check in, but before they book, or even mention it in your listing if you use instant booking.
Due to recent very wet weather, my front door started to stick and I've had a real issue with this with one of my guests. Although I did mention it to her shortly before she arrived (it had been fine the previous day), I wish that I had been able to tell her in advance. The other guests and my cleaner have been able to get in and out fine, but this particular guest simply cannot open the door, no matter how many times I showed her, to the point where I just offered to refund the rest of her stay if she left as I could see it starting to blow up into a huge issue.
Funnily enough, since we agreed that she would leave, she's been able to open the door...