New knowledge article from Claris:
FileMaker Pro 19.1.3 and FileMaker Server 19.1.2 macOS Big Sur 11.0 Compatibility
to quote it:
FileMaker Pro 19.1.3 and FileMaker Server 19.1.2 are compatible with macOS Big Sur 11.0 with no known issues.
macOS Big Sur 11.0 is not supported with earlier versions of FileMaker Pro and FileMaker Server.
We strongly recommend you use current FileMaker software.