In the Commander variant, a commander remains a commander even if it loses the creature or planeswalker card types; being a commander isn't a characteristic (C.R. 903.3) and nothing in the game can make a commander stop being a commander. Thus, in that variant, a "commander you own [on] the battlefield" can be targeted by Sanctum of Eternity's second ability even if it isn't a creature or a planeswalker.
The reason I’m asking is it turns into just a regular land