What does “I can’t do this” mean when it’s not about ability?

In this scene, one friend says, “I can’t do this. Pretend we’re just friends.” They are not saying they lack a skill. I can’t do this often means: I can’t keep living with this situation any longer, it’s too hard emotionally. Here, the speaker means they can’t keep hiding real feelings behind a “just friends” act.

Context decides the meaning. “I can’t do this math problem” is about ability. But “I can’t do this anymore,” said about a relationship, a job or a stressful routine, is about emotional limits, not skill. The topic, and what comes before it, tell you which one you’re hearing.