Details

This message is generated when the indicated limit on the variable does not evaluate to a real number or is outside of the range of machine numbers on your computer.
This message is used primarily by plotting functions.
Limits on variables in plotting functions are expected to be given in the form {x,xmin,xmax} where xmin and xmax are real numbers.
Off[message] switches off the message; On[message] switches it on. For example: Off[General::plln].

Examples

Automatic is not a valid limit for the plotting variable:

This shows valid limit specification in Plot:

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