Normal to Height - Unsupported Layer

I want to convert a normal map to a height map. but adding a normal to height filter only gives my "Unsupported Layer - Can be removed".  

Because i ran into this issue i googled on how to use this filter, because the online help is really bad. And i found a video where the normalmap is imported as texture and the filter is added and it works. 
- I only get this error. not sure if this is broken in the programm.
im using sampler 4.5.2