Reason:
17:07:25 net.minecraft.realms.RealmsBridge Realms module missing
Normally it should go to the menu, even in snapshots, and in case you run a snapshot, it'd show an error message.
Related issues
is duplicated by
Comments

No realms in or for snapshots. Only in 'latest releases' of the non pre/snapshot kinds.

why do they even have the realms button on snapshots?
Also affects 18w03b