Conditional formatting custom formula not working in google sheets

custom formula is: "=COUNTIF($B1:$B100,"month")>1."

I tried also:

"=COUNTIF($B1:$B100,"month")>1"
=COUNTIF($B1:$B100,"month")>1
="COUNTIF($B1:$B100,"month")>1"

I know it shouldn't work but I test all that comes to my mind.

I paste this in a cell: =COUNTIF($B1:$B100,"month")>1 and returns TRUE. The formula is working, the conditional formatting isn't.

Any ideas?