A TLE describes the orbit of a body at a single reference point in time, which is called the epoch. It is straightforward to extrapolate the position of the object for some time forward or backward from that snapshot, but beyond that, limitations of the propagation model cause errors to grow, and the position of the object cannot be accurately estimated.
The error message you're getting is telling you that you're using an outdated TLE. The solution (assuming you're trying to find the present location of an object) is to find a newer TLE for the object. How you do that depends on what the object is, and how you got the old TLE in the first place. It may be as simple as updating to a more recent version of pyephem.
If you’re asking for a projection of an Earth-orbiting satellite far into the future, pyephem may not be up to the task.