After extraction, check the file size. The full update for Super Mario Maker 2 (from version 1.0.0 to 3.0.2) is approximately . If your extracted NSP is smaller, one of the .rar parts may be damaged.
Here is what the 3.0.x branch (leading to 3.0.2) brought to Super Mario Maker 2 : super mario maker 2 nspupdate 302rar
Browse to the file location on your SD card and select . After extraction, check the file size
On a standard, retail Nintendo Switch, game updates are downloaded automatically from the Nintendo eShop servers. However, the search for an "NSP update" typically indicates one of two alternative environments: 1. Custom Firmware (CFW) retail Nintendo Switch