Follow path offset field can't be keyframed, generates error There is a camera following a path, and when I press I while over the Follow Path > Offset field, it throws an error of F-Curve with path 'contrains["Follow Path"].offset_factor[0]' cannot be keyframed, ensure that it is not locked or sampled, and try to remove F-Modifiers. There are no F-modifiers and it isn't locked or sampled. It has a Limit Rotation constraint on it as well but removing that doesn't help. How can I resolve this?
I downloaded your .blend file and found that if you uncheck the "Fixed Position" box, you can keyframe the "offset" value and it works fine.

Hope this helps!