As the cell value equates to an error, your condition that the cell value be smaller than 1 is not valid

Why?

When comparing, XL first compares the TYPE value of the data ( see that function for more information)


This function returns 16 for an error value and 1 for a number.
As 16 is not smaller than 1, your condition returns FALSE, the CF is thus not applied.

Instead go to Cond Formatting - New rule - select "use a formula" and enter =or(type($g3)=16,$G3<0)
and format as necessary after selecting a column range starting in G3

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