Force end of turn even if units still have move points
You're looking for the Skip Turn command:
When you select a unit, its command window will open in the lower right corner of the screen. On top of that window, you can see various buttons, including the Skip Turn button, which looks like a curved arrow.
Interestingly, according to https://www.en.magicgameworld.com/sid-meiers-civilization-vi-pc-controls-shortcut-keys/ the space key should be the proper shortcut for Skip Turn.
If you want a unit to skip many turns, you can choose the Sleep button Z or the Fortify button F instead.