Why can't my block with GUI use Itemstacks? Are there any workarounds?

Basically, i'm making a block that converts fuel items (ex. Coal, Coal Blocks, Lava Buckets, etc.) to energy. It has a GUI with nothing more then a single input slot and some text. It has energy and GUI turned on with the GUI linked. I made a small little procedure for it that should let it convert the items to energy, but because it uses Itemstacks it can't be selected for the block. Are there any fixes or workarounds?

P. S. - I'm a noob at this. Sorry if I missed something super simple that would've fixed it.