/execute command only working through chat, no command block output.

I am trying to use the command /execute if block 212 5 -141 minecraft:diamond_block run fill 212 5 -141 212 7 -146 diamond_block replace air in a command block, however for some reason I can't work out why it doesn't accept the command. If I do the command in the chat menu, it works however when I use it in a command block it doesn't and the only message that I get is an arrow pointing to the end of it with Here written. I've checked gamepedia and it seems like it's fine.