There is an effect that forces opponents creatures to attack and I control propaganda. Opponent chooses to attack me and chooses not to pay the tax. Are they forced to attack someone else?
Any requirements to attack must be met, if possible, or as many as are possible to meet. A player does not have to pay any costs in order to make a creature able to attack, but any that is must attack a valid player or planeswalker, unless none exist, in which case, the creature does not have to attack.
Note, that the choice to attack with a creature and whom to attack is made before paying any costs to do so. So if the player chooses a creature with an attack requirement to attack you and gets to the point where he has to pay mana to do so but refuses to pay, the attack declaration is illegal, and the game rewinds to the beginning of the declare attackers turn based action, and he tries a different set of attackers, until a legal declaration is made.
It depends on the effect. In general, if you control Propaganda and:
...the effect requires the creature to "attack if able" (and no other requirements apply to that creature), the creature must attack another player or a planeswalker if able if the creature doesn't attack you because the opponent declines to pay the Propaganda cost (C.R. 508.1d).
...the effect requires the creature to attack you if able (and no other requirements apply to that creature), the creature is not required to attack another player or a planeswalker (if any exist) if the opponent declines to pay the Propaganda cost (C.R. 508.1a, 508.1d).
Note, that the choice to attack with a creature and whom to attack is made before paying any costs to do so. So if the player chooses a creature with an attack requirement to attack you and gets to the point where he has to pay mana to do so but refuses to pay, the attack declaration is illegal, and the game rewinds to the beginning of the declare attackers turn based action, and he tries a different set of attackers, until a legal declaration is made.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
EDIT (Apr. 28; Oct. 23): Correctness edit.
EDIT (Jan. 2, 2019; Jan. 11, 2019): Correctness edit.
EDIT (Apr. 10, 2019): Edited.
EDIT (Apr. 11, 2019): Correctness edit.