delete specific character "[" from a file

Yes, the [ character is special as it starts a [...] group (a bracketed expression).

With sed on OpenBSD, your command gives a more helpful error message:

$ sed 's/[//g'
sed: 1: "s/[//g": unbalanced brackets ([])

To delete all [ characters using sed, escape it:

sed -i 's/\[//g' file

Or put it inside a bracketed expression:

sed -i 's/[[]//g' file

Or, use tr,

tr -d '[' <file >

Also, don't use in-place editing with sed until you know the expression that you are trying out actually works, or you will possibly have to restore your data from backups.