In order to emulate the Spectrum Next, you need a few bits and pieces:

Firstly you need the official Next distribution from https://www.specnext.com/latestdistro/

At the time of writing, you need the sn-complete-24.11.zip which you should extract to a folder that ZXSpin can reach.
Once extracted, switch to Spectrum Next emulation, and point ZXSpin's DivMMC ROM to Machines\Next\enNxtmmc.rom, and mount the folder you extracted to as the SD Card.
The actual machine ROM needs to be set in the Hardware options/ROMs tab - use the enNextZX.rom in the same folder as the DivMMC.rom file.
After that you need the boot ROM from https://www.specnext.com/forum/download/file.php?id=1164
Unzip the files from tbblue.zip into the ZXSpin ROMs folder.

Then you should be able to start Spectrum Next emulation.