While Basandra, Battle Seraph is on the battlefield, players can’t cast any spells during the combat phase, including permanent spells with flash.

Players may still activate abilities during combat.

If, during a player’s declare attackers step, a creature is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under that player’s control continuously since the turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having a creature attack, the player isn’t forced to pay that cost, so it doesn’t have to attack in that case either.