When this message pop out, with emulation running, go to Config => GameCube and change Slot A to "none", apply, change it back to "Memory Card" and apply again. This should let you save again in SMS... Remember, emulation must be running, otherwise it won't work...
Avell A70 MOB: Core i7-11800H, GeForce RTX 3060, 16 GB DDR4-3200, Windows 11 (Insider Preview)
ASRock Z97M OC Formula: Pentium G3258, GeForce GT 440, 16 GB DDR3-1600, Windows 10 (22H2)
ASRock Z97M OC Formula: Pentium G3258, GeForce GT 440, 16 GB DDR3-1600, Windows 10 (22H2)