Fill Series is not showing up as an option

[mishh]

I have a code that is 4 letters followed by 16 numbers. I used to be able to use fill series to make a list of these numbers going up by 1 each time. Now all of a sudden fill series isn't an option.

HELP!

I tried playing around with other letter and number combinations, and if I only have 10 numbers, fill series works, but with 11 or more, it doesn't. Unfortunately I need 16, so I need this to work.