No. Series Automatic Generation

Hi,

I am facing a problem, when i open any old PO and press F3 on No. field, it is not picking up the next no. series. It is showing an error:
"It is not possible to assign numbers automatically. If you want to assign the nos automatically, please activate Default Nos in No Series D"

Eg:
There are 4 No. series when i look up the No. field.
1. A
2. B.
3. C.
4. D.

I am pressing F3 on any No. series, it is showing the same error referring the D No. Series. I need to autoincrement the B No. series, then system must do it, as Default Nos, is already Yes for it.