Can't open A/R Invoice

Hi, SAP Business One, 2007A SP:01 PL:11.

When I try to open A/R Invoice I get the error message "You must first define a numbering series in Administration". We have about 10 users on the system and only one of them can open this form. I can go into numbering and define it but for what values? How do I enable a user to access this form?

Thanks!