It's already explained in the ranking criteria, for maps with certain drain time:
If the drain time of each difficulty is...
...lower than 3:30, the lowest difficulty of each included game mode cannot be harder than a Normal.
...between 3:30 and 4:15, the lowest difficulty of each included game mode cannot be harder than a Hard.
...between 4:15 and 5:00, the lowest difficulty of each included game mode cannot be harder than an Insane.
Difficulties below the highest difficulty can combine break times with drain time to meet the above thresholds. This does not apply to difficulties with less than 30 seconds of drain time. If the lowest difficulty of the beatmap is a Hard or Insane, judgement regarding the suitability of its gameplay elements is up to the Beatmap Nominators and Nomination Assessment Team members for the corresponding game modes.
osu!mania beatmaps are bound by a different set of drain time thresholds and other spread rules defined in the osu!mania ranking criteria.
You can rank single difficulty map, but it needs to adhere the rule too.