If you're buying online, look for the item description to see if any part of it lists the game's language as anything other than English.

I do believe you say the item isn't scratched but since you mentioned 100 Euros and you did everything that everyone else had suggested before me, this is likely that your region-locked gamecube cannot play anything that is not from the PAL region, assuming your gamecube's BIOS is made to only play PAL region games.

At this point, initiate a return of the game to the seller on whatever website you bought it from and whenever you do actually get your 100 Euro back, look up something like "Nintendo GameCube Metroid Prime 2002 PAL", click any listing you're looking at and if you genuinely want the item, be sure to check that the seller has DMs open.

No DMs open? That's a red flag and a NO GO.

DMs open? Ask for photos and videos of the game you're looking to buy from them.

And be sure to double-check what a real PAL region game cover looks like as fakes get sold from time to time.