I have just found the solution after struggling with this for several years now and gave up after receiving an incompatible floppy emulator, wrongly shipped I think because I ordered the correct one. The correct emulator for the Casio CTK-811 is a Gotek SFRM144-U100K. However, the jumpers need to be installed before it will function correctly. Download the user manual from Gotek and you will see the jumper locations illustrated MO S1 S0 JB JC JA J5. The emulators is generally shipped with S1 in place and J5 parked on the common side bridging J5 & JA. Unpark that jumper and set it to the J5 active (parallel orientation with the S1 jumper). So now you should have S1 & J5 jumpers set. Pin 1 of the ribbon cable to the floppy drive has the RED stripe so make sure you get that right. The infuriating thing is that Gotek gives no indication whatsoever about the function of J5 as though it doesn't matter but it does matter - big time! Without J5 you will get the error message from the CTK-811 "ErrFormat". To load files to the stick and format it from a Windows 10 computer etc. in the correct floppy file structure, you can download free software from the German vendor "ipcas" Germany - English version available.
