Probably already been answered but I will try anyway.
I can't speak for the Explorer but my 14 Taurus SHO and pretty much every Taurus from 2013-2016 had the card in the center console. I am taking a hopefully decently educated guess that with NAV you have the SYNC My ford Touch (MFT) and SD card is probably same location. SD slot should be next to the USB Ports.
NAV SD CARD FAULT: This is usually a fairly simple fix. Remove for about 30 seconds minimum then re install until it clicks.
I noticed the LAGOS in the profile, if it is Lagos, Nigeria then follow above steps.
If vehicle is in the US especially during extreme cold and dry air that message can come up fairly frequently. The common denominator is usually extreme cold (below 20F) and extreme dry air when it gets too cold to have precipitation. The trick for me that worked pretty well was remove the card, warm up in your hands for a couple of minutes, make sure there is no contamination on the card especially the metallic contact points and re install. The fault should clear it might take a few tries of insert, remove, insert, remove etc...
