Hi Mike,
Split Text works with InDesign 2024. Try closing InDesign before installing it and check that you're running InDesign as adminisrator:
Here is information on how to run InDesign as administrator on macOS:
The installer worked for me. Are you on macOS? You may have the setting to not allow installation from non-Apple developer selected.
For anyone with the same issue, it may be due to InDesign being installed in a non-standard location. If this is the case, then you can try using one of the following non-Adobe-built Extension Managers to install the .zxp file (in the assets directory).