Can't create Custom Resolution

Hello everyone,
I am working on a project that requires a custom resolution. The only trouble is I can’t edit the resolution presets on the welcome screen, or in the Storyboard->Properties->Project Resolution page. To clarify, there is no ‘+’ sign on the welcome screen to add a new custom resolution preset. Any help would be appreciated.

Enter the values of your custom resolution values and
enter a unique name for this custom resolution and the
“+” sign should appear.