Hello,
Blue Prism utilises a Standard API used by most enterprise strength emulators called Hllapi. I have installed your Mocha mainframe and I can find no reference to Hllapi in the documentation or product settings. There is also no hllapi or ehllapi or winhllapi dll file distributed with the product.
It might be worth contacting the vendor, but I think it is most likely that Mocha is not a hllapi complient emulator. Your options are therefore to either change to use another emulator that is hllapi complient (which is what I would recommend) - they all will be able to interface with TN3270 mainframes. The other less attractive option would be to create a Surface Automation interface to interact with the mainframe, there is a guide to how to do that in the learning area of the portal.