To refer to singular/plural items without knowing the actual quantity [duplicate]
For the second variation (and there's nothing wrong with at least one), use the indefinite article a; that's what it's meant for:
- Make sure a file exists.
The fact that one file exists doesn't preclude the possibility that multiple files exist. But having at least one exist (which a covers) is a necessarily condition, and it's all that's required to avoid the error message in the question.