I would try 2 things:
1 - On the PC with the SD card inserted: Go into disk manager and verify that there are no hidden partitions on the SD card (I had this problem, even after formatting, as the hidden partition was still there and the Nextion cannot deal with multiple partitions)
2 - Get and use the SD Card Formatter.? It is free on line from the SD consortium.
https://www.sdcard.org/downloads/formatter/
If the Nextion still does not see the card, then your only choice is to get another card, or another Nextion.? Just being able to read the SD card in the PC does not mean that the card is OK to use in the Nextion (They must have a really poor interface driver).?
Above worked for me, YMMV
73
Evan
AC9TU
