The new Minecraft command parser thinks a part of code is wrong when it really isn't

The command breaks blocks in a 3x3 block range under your feet if you are standing on this pattern

Command works flawlessly, but the parser thinks it's incorrect and won't work

I think it might be upset at the "at @a" part allowing multiple entities. If you put the command into chat what message do you get? (EDIT: That wouldn't explain the command running fine though...)

No, that's not it, as /execute at allows multiple targets.

When I tested the command you put in the description, no syntax errors appeared. I can imagine that there was a zero width space or another quirky character that the command parser couldn't handle?

Possibly that, or even just 2 spaces (MC-121859 / MC-65034), though I can't see it if so. I do also see that you have some kind of mod installed so that may also be at fault (can you reproduce in vanilla?)

What does happen when you copy and paste this command from here?

execute at @a if blocks 209 78 -131 211 78 -129 ~-1 ~-1 ~-1 all run fill ~-1 ~-1 ~-1 ~1 ~-1 ~1 stone destroy

I'm soooooo confused!!!!

@violine1101
This:

Same thing

Huh, now I get the red text too after copy pasting from violine, seems to differ per session.

Please check if that's still an issue for you in the latest 1.15.1 stable release or the latest 1.15.2 (pre-)release.