Hi,
This is a UE5 standard n, not Fortnite/UEFN, the error is a blueprint error
The code is an Assert Error which is used to tell the user a supplied value from user project has a problem.
In this case this is a the assert RequiresCachedVariable which where converted to normal text is
Assert Cached Variable is Required.
When using UEFN, recommends using
Unreal Revision Control Best Practices in Unreal Editor for Fortnite| Epic Developer Community
Normally this would fix your work, however, you need to find a backup of your project