@Billie37 No it's not normal and you should not accept it, and you should not consider for one moment allowing anyone to access your home without you there- these are complete strangers to you- would you give any other stranger the key code and let them enter your property?
Guests may ask to preview as an excuse for stealing, or scoping the place out for a future theft, or they may use the excuse to talk to you in person to try to get you to book off-platform. Some guests may have nothing nefarious in mind at all, but simply don't understand that this just isn't done.
Guests should be able to ascertain from your photo gallery, your listing description, and asking you any questions they need to before booking, to make sure this is a place they want to book.
Just politely tell them that, and that previewing a place they book on Airbnb isn't something Airbnb hosts allow and that if they need any questions answered about the listing, you'll be happy to address them.