The usual cause of this problem is that the `dash` symbol has not been selected after the first part of a game ID. For example, you may have been trying to enter APLP406891, which is incorrect, instead of APLP-406891, which includes the dash symbol.

The required dash symbol is selectable from the onscreen keyboard which is shown when you are viewing the game ID entry screen.

Please be aware that the version number shown on the front of a game cartridge (for example, NTR-YH2P-EUR) is not a game ID.