Your page says
in the Invoice no. field, enter the custom number or letter sequence you want to use for your forms. The sequence you set here applies to all sales forms.
QB will NOT increment a letter sequence, there must be a number
Thanks for the callout about not being able to increment numbers, Rustler. I'll make sure that the content team clarifies that!
Info spot on, plus that helpful YouTube video dialogue box stayed open even when I changed screens within QuickBooks.