What is your macro?
I've used macros like the one below to overcome fizzling heals, and it spams the message, but that is less important than having a missed heal.
I don't think you can separate out the "cannot use that command while casting" since that is activated by trying to cast a spell and is returned instead of the casting message. Without having the options menu in front of me, I think you would need to hide all your casting messages to hide it.
/g Complete Heal
/cast 1
/cast 1
/cast 1
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