You decide whether to have the targeted creature block Turntimber Basilisk this turn if able at the time the landfall ability resolves, not at the time Turntimber Basilisk attacks.

Deciding to have the targeted creature block doesn't force you to attack with Turntimber Basilisk that turn. If Turntimber Basilisk doesn't attack, the targeted creature is free to block whichever creature its controller chooses, or block no creatures at all.

If Turntimber Basilisk's landfall ability triggers multiple times during the same turn, you can have multiple creatures block it that turn if able.

Tapped creatures, creatures that can't block as the result of an effect, creatures with unpaid costs to block (such as those from War Cadence), and creatures that aren't controlled by the defending player are exempt from effects that would require them to block. Such creatures can be targeted by Turntimber Basilisk's landfall ability, but the requirement to block does nothing.

If you choose to have the targeted creature block but that creature isn't able to block Turntimber Basilisk (for example, because the targeted creature can block only creatures with flying), the requirement to block does nothing. The targeted creature is free to block whichever creature its controller chooses, or block no creatures at all.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).