@pgoellner in your screenshot you use reprapFirmware 3.2Beta4.1, but for 3.1.1 you need a different version. Go to github, go to tags, download the zip of tag version 3.1.1. This version will fit to the 3.1.0 libraries. DuetWifiSocketServer is not needed for compilation.
If you want to use the newest beta build instead, I would choose a version which is tagged for RRF and all libraries. CoreNG is replaced by CoreN2G, so this is probably to be used instead for the newest builds.
When compiling, is is sufficient to set the target in RRF, clean the project and compile. It will start compiling all libraries with the target and with the original set targets of the libraries (a bug), then RRF.
