Details

This message is generated when the indicated expression is expected to be an algebraic number.
The value of the Extension option in Factor and related functions is expected to be an algebraic number or a list of algebraic numbers.
An algebraic number is a number that can occur as the root of a polynomial with integer coefficients. Integers, rational numbers, roots of integers, and Root expressions are algebraic numbers.
Off[message] switches off the message; On[message] switches it on. For example: Off[General::nalg].

Examples

All is a symbol rather than an algebraic number:

This shows a valid value for the Extension option in Factor:

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