When you build your program, does the resulting folder contain the EpicorRestAPI.dll? If so, drag that into the server assembly folder just like your program. It can’t find it when your program references it.
Is all you are doing running a BAQ? You can query data directly in a BPM using Linq like the solution here, and if all the external assembly does is call BAQs, it is over-complicating the process significantly.