The firework you used was once labeled “skinny” (which lasts about 60 seconds per use). They have had a history of harming (especially crashing) the server, and they cannot be created in Project City Build.
I remember previously discussing with you about using harmful items and why it is forbidden from use on Project City Build. You were also previously told not to use such NBT Items, so to catch you using that firework warranted your ban.
I still have yet to hear the name of this “staff member” who gave it to you.
Regardless, just because someone gives you an NBT item of that type does not mean you can use it, and it especially does not set you free of responsibility.
Because this is your first ban, it will expire in 24 hours.
Until then, I strongly recommend rereading and following the rules.
When your ban expires, make sure your inventory (and chests) are clear of such items.
Remember that further violations will have consequences.