How can I get more Companions quests after encountering a glitch at the Rift Watchtower?

This is a known bug according to the UESP wiki's Rift Watchtower article:

• The bandit leader (an unnamed Orc) does not respawn, which makes it impossible to complete radiant quests at this location if he has been killed previously.
  • This bug is fixed by version 2.0.1 of the Unofficial Skyrim Patch.

PC, PS4 and Xbox One players can install the Unofficial Skyrim Patch (original / Legendary Edition - PC) / (Special Edition - PC) / (PS4) / (Xbox One) to fix this bug. Related USKP changelog (v2.0.1 - 2014-02-08):

The orcs used at Rift Watchtower did not respawn properly which would lead to broken radiant quests at this location. (Bug #14618)

Since the Unofficial Skyrim Patch is not available on the Nintendo Switch*, and you can't use console commands on the Switch to finish the quest, you'll have to load an earlier save before you accepted the quest to kill the bandit leader of the Rift Watchtower. Then, try to get another Companions radiant quest without the Rift Watchtower as the target location.

---

^{* "Porting this mod to a platform where modding is not officially supported or legally allowed is strictly prohibited. This includes, but is not limited to, Nintendo Switch..." - Unofficial Skyrim Special Edition Patch: Readme}