That message is a notification about your account's storage limit, not a warning that you are currently out of space.

The message is simply telling you the total amount of data your account can hold, which is 50.0 GB. The line "Upgrade your account to get unlimited storage" is a standard marketing prompt to encourage you to switch to a higher-tier plan. It's appearing because your account has a capped limit, even though you have only used a small fraction of it (4.0 GB).

Essentially, it's a promotional message that shows up for all accounts with a storage cap, regardless of how much data they have used. Your account is functioning normally.