Solution
The main reason a model errors out with the "[Synth 8-5809] Error generated from encrypted envelope" message is because, as explained in the Known Limitations for the
HDL Coder Support Package for NI FPGA Hardware, purely combinatorial circuits such as circuits without clocks are not supported.
Even the simplest model must include a clock or time signal to be recognized as a valid model by the package. If your model does not use any of these signals, NI recommends adding a block that has a SampleTime parameter, such as the Delay block. See below a model that just has a gain stage with one of these blocks:
After adding the Delay block, the model was compiled successfully.