Spells have a minimum mana requirement. That notice simply means you typed in 'prep spell 1' when the minimum mana was anything more than 1.
So for example if you type in 'prep/target spell 1' and the spell costs 5 mana to cast normally, the game will auto bump you to preparing it at 5 mana. That's what that messaging means.
You can see the spell mana requirements by using discern (discern spell), or by looking on the wiki
