NFC on iPhone is finicky. Most issues come down to hardware, iOS settings, or tag positioning.
Hardware requirement
NFC scanning in MapVS requires iPhone 8 or newer. Earlier models have NFC hardware but not the background-scanning API MapVS uses.
NFC must be on
In iOS Settings > NFC. Should be always on for iPhone XS and later; on older models it may need to be toggled per-session.
Tag position
Hold the top edge of the iPhone directly against the tag for ~1 second. Cases with a metal layer will block reading - remove the case if in doubt.
Tag type
MapVS reads NTAG213 / NTAG215 / NTAG216 (any standard NFC sticker). Older tag types (Mifare Classic) don't work. If in doubt, test with the free NFC Tools app - if that reads it, MapVS will too.