Hi Ash,
TDA Rule #3: Official Terminology and Gestures
Official betting terms are simple, unmistakable, time-honored declarations like bet, raise, call, fold, check, all-in, complete, and pot (pot-limit only). Regional terms may also meet this test. Also, players must use gestures with caution when facing action; tapping the table is a check. It is the responsibility of players to make their intentions clear: using non-standard terms or gestures is at player’s risk and may result in a ruling other than what the player intended. See also Rules 2 & 42.
Was the dealer absolutely certain that Player A tapped the table? Or, did the dealer say something like: "I thought Player A checked?" Did other players at the table see Player A tap the table? Does Player A have a reputation for nervously tapping the table, keeping beat with music on their ear buds, has a physical ailment?
Player A wasn't facing a bet and action was heads up. Without clear evidence to the contrary, I'd rule that Player A didn't check and is allowed to bet.