Raids cannot spawn inside of or on string, including strong that's part of tripwires.

How to reproduce
1. create a superflat world and a village (villager, workstation, bell)
2. run /fill ~90 ~ ~90 ~-90 ~ ~-90 minecraft:tripwire
3. run /effect give @s minecraft:bad_omen
4. you'll notice the raid fails to spawn due to the string.
5. if you remove the string the raid spawns as intended.

MC-258425 supports that mobs should spawn in string/tripwires.