David McNamara had left his coin in the dressing room when officiating Manchester City's clash with Reading on October 26.
So instead of a coin toss with the two captains to decide which team kicked-off, McNamara chose the rock, paper, scissors game.
The laws of the game state that a toss of the coin has to be used.
McNamara will be suspended from Monday, November 26, to Sunday, December 16.
He is a 2b level official which sees him referee in the National League North, Women's Super League and the Football League Under-23s, while he is a fourth official in the EFL.
Last Sunday, McNamara was fourth official for Chorley's 2-2 draw against Doncaster at Victory Park in the FA Cup.
In pre-season, he refereed Preston's friendly against AFC Fylde at Mill Farm.
A spokesperson for the FA said: "The FA can confirm that referee David McNamara has been suspended for 21 days, starting from Monday 26 November, after accepting a charge of 'not acting in the best interests of the game'.
"This follows an incident in the FA WSL match between Manchester City and Reading on Friday October 26 when he failed to determine which team would kick off the match by the toss of the coin, as required by the laws of the game."