There is no “there could have been a memo”, it either existed or it didn’t. Comey testified under oath that it existed. Regardless of the ability to produce it, Comey can be charged with leaking and likely destruction of government property if he can’t produce the document. If he admits the memo was a fabrication he then gets with perjury.
Comey’s either a liar, or a liar. Bottom line....he’s a liar.