The error message indicates that the vacuum energy parameter is above the vacuum potential at an infinite distance from the film. In the old FLEUR this parameter is set in the enpara file.

If you, however, calculate a metallic film of such a thickness, it is probable that this is due to charge moving around in the film. This can often be overcome by performing some scf iterations with linear mixing and a very small mixing parameter in the beginning. A more elegant and efficient solution would be the use of the Kerker preconditioner, which is available in newer FLEUR releases, but not yet in version v0.26.