Nemesis: Retaliation · Board game rules
How do you complete a solo/co-op game when you cannot complete it?
Quick answer
There's no special "completion" rule for Solo/Coop — the game simply ends via the normal End of Game triggers, and if the Mission Tasks aren't fulfilled (or no Character survives), the group loses.
Details
In Solo/Coop mode, winning requires that all Solo/Coop Mission Tasks be fulfilled and that at least 1 Character Survives [p.40 — Additional Modes]. If any Mission Task requires multiple copies of the same requirement (e.g. gaining a Data token), that many Characters must fulfill it and Survive [p.40 — Additional Modes].
The game itself always ends the same way as in the standard mode: either when Round 14 ends (all Characters who haven't Escaped/Hibernated are considered dead) or when all Characters have died or Escaped/Hibernated [p.7 — Endgame Overview] [p.39 — End of the Game]. There is no separate "failure" procedure — you simply run out of Rounds or Characters before satisfying the win condition, which means the cooperative group does not win.
If you find you cannot possibly complete the Mission Tasks anymore (for example, a required Room or token has become permanently inaccessible), the rulebook does not describe an early-loss declaration; players should continue playing until one of the two End of Game triggers occurs, then check the win condition as written.
Watch out: Difficulty can be tuned by changing the number of Mission Tasks during setup [p.40 — Additional Modes], so if your group finds the Tasks impossible to complete, consider adjusting this before your next game rather than expecting an in-game "concede" mechanic — none exists in the rulebook.