According to a post by u/bowtiesareswaggy, “What the title says, and this is even after clearing more of my storage. I have 16.34 GB available out of 64 GB. Tapping the warning also results in no reaction whatsoever from the OS. This was a problem through every iOS 15 beta version I was on, and now I am on the RC version and it is still not fixed.”
Other users are chiming in with similar issues where they say that they have been given the same warning even though many of them have a lot of space leftover. Some users have tried resetting their phone settings but it seems that did not solve the issue. Some have even tried erasing their phones completely and factory resetting it, but to no avail.
That being said, this seems to be more annoying than it is breaking. We imagine you should still be able to go about your business and can safely ignore the message. Apple hasn’t acknowledged the issue yet but hopefully they are aware of it and that they’ll be issuing a fix soon.