If this is a fresh installation, Tags should be enabled by default. If that's not the case, I recommend deleting the forum and re-starting the installation. The installation probably failed part-way and this is likely not the only broken thing.
Otherwise, we'll need to know what happened earlier. Did you disable Tags yourself originally? Did you install other extensions and/or used the forum while Tags was disabled?
There should be details about the error you encountered in the Flarum log file. See https://docs.flarum.org/troubleshoot for where to look.
As there are many (third-party) extensions that depend on Tags, it's often not possible to disable or purge Tags at all. If you manage to press purge in that situation, an SQL error is likely to occur when Flarum tries to delete the Tags table that other extensions still rely on. If you pressed purge in that situation, repairing the database by hand will probably be too complicated, you should consider restoring a backup.
If you didn't disable Tags yourself, you might have installed an incompatible combination of extensions, and the automated extension dependency check forcefully disabled Tags. If this happens, you might be stuck unable to re-activate it until you remove the other problematic extensions. But if this is the case, there should be a clear error message about the extension dependency error when trying to re-activate.
