You’re gonna need to include a bit more details if you’d like more accurate information. But as long you aren’t allowing players to purchase a loot crate for Robux or any currency that’s obtainable via Robux you don’t need to disclose the chances.
E.g a player logs in daily and get a free loot crate; This crate is unobtainable in any other way period.
An example where chances would need to be disclosed is.
E.g a player logs in daily and get a free loot crate; This loot crate is obtainable via robux or some in-game currency you can obtain in exchange for Robux.
The difference is in the first example the ONLY way to get a loot crate is by logging in regularly. Where as the second example you can obtain one via purchase or exchange in addition to logging in regularly.
If it’s not connected to a currency you can exchange for Robux then you don’t need to disclose the chances. Though I’d recommend to think about how you can include exposing the chances with the mechanic as part of the gameplay instead.
E.g with this crate you have a 20% chance of an epic. Just so people know what things they could get.