FlexiSprite supports Built-in, URP, and HDRP. The shader files must be built for the same pipeline as the Unity project.
Set the active pipeline
Open Tools > AnimatedSprite > Master Setup. In the state bar, compare Unity's project pipeline with FlexiSprite's shader build.
If they do not match, press Set Target to Project Pipeline, then Full Rebuild for Project Pipeline.
Import matching demos
Use Import Demo Packages for Project Pipeline. This imports demo scenes that use the current pipeline variants.
Do not mix URP demo materials into a Built-in project or Built-in materials into a URP project.
Pink materials usually mean the project pipeline and shader build do not match. Rebuild first, then migrate materials if needed.