can fuel for the cause counter spells already in the game?

[Neil]

can you use a fuel for the cause to counter a spell thats already in the game?

[Kenji]

No. A spell is an object on the stack that hasn't resolved yet. Once something is on the battlefield, it is a permanent, not a spell. As such, it's not a legal target for Fuel for the Cause, or any counterspell.

Here's the Comprehensive Rules excerpt to help you out, as well:

111.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the
card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually
its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves
(see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves
the stack. For more information, see section 6, “Spells, Abilities, and Effects.”