"無" is a formal written Chinese.
"冇" only exists in Cantonese writing.
Both 無 / 冇 = nothing, do not have, negative.. ( opposite word of "有", have, exist)
Both words have the same pronunciation.
When describing "do not have", "no exist" in Cantonese , you can either use 無 or 冇, but 冇 looks more "native".
Example:
1. nobody
冇人
2. no problems
冇問題
3. do not have time
冇時間
4. do you have time? ( 有冇 both negative+positive implies questing )
你有冇時間?