Format Rule Stopped when changing Index Key

[Paco_Delacroix]

Hi,
Probably I am doing something wrong that I can´t figured out.
I have a text column with different values everything works ok until i change the rownumber key to another column key,
Thanks in advanced

[Steve]

In what way aren’t things working?

[Paco_Delacroix]

Thanks for your response,
With Key Column marked to Codigo de Tarea format rule does not work:

!

With Index Key marked to _Rownumber format rule works ok.
appsheet1b|690x284

Do these images ilustrate the problem?

Thanks

[Steve]

What is the format rule expression?