Hi Ivan,
The error you posted is because the format of the json file Spine exports has changed, likely due to new features and data added/removed in the Spine v4 export format.
However the functions in SpineSlotExt were added natively into GMS2 way back in v2.2.2, so it is no longer needed in any versions of GMS2 later than that as the functions the extension added (ie. slot colouring) are now "built-in" and a native part of GML and can be found in the manual here.
This information is also noted at the top of the main page.