Could be a mismatched version — the version of zerotier.npk copied MUST match the version of RouterOS installed (now, once installed the package will get automatically upgraded in future).
Or, more likely, it’s out of disk space — it has only 16MB for packages. So there may have an auto backup file, dunno. But any file on the device have to be deleted before copy the zerotier.npk & rebooting.
If either don’t work, the log still show what happened. If logs shows nothing after a reboot, you didn’t copy the file to the root of Files.
Try netinstall, sometimes it clears out space which is left behind from earlier versions.
If that doesn’t help, try 7.14rc1, it should be a bit smaller (maybe just enough to have zerotier added).
But why zerotier ? Wireguard is also an option and default available in ROS7.
If you go to files and remove any files under /flash, it should fit when trying upgrade. You shouldn’t need the wireless or wifi-qc-comm on LHG LTE18 – so if present in /system/package uninstall them.
Or you try, “testing” channel in /system/package, perhaps 7.14rc will work.