Is there another word for unpreparedness?

"Unpreparedness" seems clunky, and thesaurus.com wasn't of much help. Is there another word that might do, for example, in this sentence? "Having seen everyone pull out their tents, Jack twiddled his thumbs, acutely aware of his unpreparedness."

In this sentence, I find "unpreparedness" unsuitable because it just feels as though it's not a real word, as though someone just added "-ness" to "unprepared".

Your use is correct. That said, I agree does affect the flow of the passage. Depending on your writing style, you could consider:

"Having seen everyone pull out their tents, Jack twiddled his thumbs, acutely aware of his wanting preparation."

wanting - adjective: lacking in a required or necessary quality.

Or, less whimsically:

"Having seen everyone pull out their tents, Jack twiddled his thumbs, acutely aware of his inadequate preparation."

inadequate - adjective: lacking the quality or quantity required; insufficient for a purpose.