It’s a language localization problem. The material pin name has been incorrectly localized in 5.6, which breaks the compilation.
You can either try to switch the editor to English or if you can work a little with source code, apply a simple fix which is already on the UnrealEngine github:
https://github.com/EpicGames/UnrealEngine/commit/c7c8a29e9cb03a47090877405b6102d3309dcb19
I suppose it will be included in a 5.6.X release as soon as possible.