The Rulebook says it's a 15-yard penalty. I have no idea why the refs didn't call it.
Section 9 Instant Replay
The League will employ a system of Referee Replay Review to aid officiating for reviewable plays as defined below. Prior to the two-minute warning of each half, a Coaches’ Challenge System will be in effect. After the two-minute warning of each half, and throughout any overtime period, a Referee Review will be initiated by a Replay Assistant from a Replay Booth comparable to the location of the coaches’ booth or Press Box. The following
procedures will be used:
Coaches’ Challenge. In each game, a team will be permitted two challenges that will initiate Referee Replay reviews. Each challenge will require the use of a team timeout. If a challenge is upheld, the time out will be restored to the challenging team. A challenge will only be restored if a team is successful on both of its challenges, in which case it shall be awarded a third challenge, but a fourth challenge will not be permitted under any circumstances. No challenges will be recognized from a team that has exhausted its time outs. A team that is out of time outs or has used all of its available challenges may not attempt to initiate an additional challenge.
Penalty: For initiating a challenge when all of a team’s time outs have been exhausted or when all of its available challenges have been used: Loss of 15 yards.