If, during your declare attackers step, Juggernaut is tapped, is affected by a spell or ability that says it can’t attack, or you haven’t controlled it continuously since the beginning of your turn, then it doesn’t attack. If there’s a cost associated with having Juggernaut attack, you aren’t forced to pay that cost, so it doesn’t have to attack in that case either.

If there are multiple combat phases in a turn, Juggernaut must attack only in the first one in which it’s able to.