Here's why:
* "Is to" is not a complete verb phrase. You need a helping verb like "am," "are," "was," or "were" to make it a complete verb.
* "All soon" doesn't make sense in this context. It seems like you're trying to ask if something is correct, but the phrase "all soon" doesn't convey that meaning.
To make the sentence correct, you need to rephrase it. Here's an example:
"Is 'all' the correct word to use in this sentence?"
This sentence makes sense and is grammatically correct.