If I didn't have mobile number, can I apply a Google voice number? when I try to apply a Google voice number, it always ask me the mobile number.
According to this Google Voice FAQ, you need an existing phone number. It doesn't have to be a mobile number, but you need to have access to a phone that is not currently associated to a Google Voice phone.
Speculating on my part, this is at least partially to avoid people making large numbers of Google Voice accounts for spam or other purposes.