activate "create custom format"

Have you selected a cell or cells first?

You could post a screenshot of what you are trying to do (shift-command-4, use "cross-hairs" to select area on screen, release, start a new post here and insert the screenshot image from the Desktop with the "mountains and moon" icon below the area where you are typing the text of your post).

SG

jdiazh wrote:

t only happens with this file. With others it's fine.

Ah, then perhaps that one file is somehow corrupted.

Or was it perhaps imported from Excel. That can sometimes result in mysterious problems.

If it was working before then you could try going back to an earlier version at File > Revert to > Browse All Versions ... Or Time Machine backup.

Or try copy pasting into a new document and see if you can get the format to work there.

SG