unable to activate

@bomietoor Did you try completely uninstalling both ZenCache and Comet Cache and then trying again?

If you still get a fatal error, I suggest enabling WordPress debugging to see if you can get some more information about what the error is: https://codex.www.ads-software.com/Debugging_in_WordPress

if I uninstall ZenCache , and install again. how to keep the settings from previous zencache ?

@bomietoor If you leave the Plugin Deletion Safeguards enabled, then the ZenCache settings stored in the database won’t be deleted when uninstalling and when you install Comet Cache, it will detect the ZenCache settings in the database and import those.