I have a spreadsheet that has been in use for several years and has plenty of
formulas and information. Trying to improve this sheet I came up with the
idea of coloring some cells according to some criteria, so that the users
easily distinguish special conditions of some values.

The problem is that when I tried to apply the conditional formatting to
those cells, the option Conditional Formatting is greyed out in the Format
menu.

Any ideas on how to fix this?

Thanks in advance

Jorge Jaramillo