NOTE: The question originally did not include the sentence that provides context. Given that context, "unless" is the only sensible choice. I am leaving this answer up to illustrate some of the options that would be available if the context was not provided.
The other respondent's answer (unless) is likely the expected one. There are other possibilities that make some kind of sense, and would be perfectly grammatical:
You can't learn a musical instrument unless you practise every day."
If you don't practice daily, you will not learn to play. This is probably the "correct" answer.
You can't learn a musical instrument if you practise every day."
Your mind needs to rest a little - take occasional breaks from practicing if you wish to learn faster.
You can't learn a musical instrument because you practise every day."
Similar to the previous one - daily practice is hampering your progress.
You can't learn a musical instrument until you practise every day."
You have been wasting your time with occasional practices. Change your behavior and start practicing more, or you won't learn.
You can't learn a musical instrument even if you practise every day."
You are a fat-fingered, tin-eared incompetent. You will never learn to play an instrument. Give up now. Although would also work here.