OK well which airport for a start? Or, is it any airport?
In the "Postion Initialization Page" you enter the ICAO airport designation for your point of origin. If you get a message that says "NOT IN DATABASE" it means just that. If you think it should be in the database then maybe you need to update your AIRAC cycle.
Typically, from experience, airports which would not normally take 737 size a/c are likely to be excluded from the database. Also, obscure airfelds or military airfields may be excluded. I found that virtually all major airport in China were at one point "NOT IN DATABASE" but maybe that has changed since.
Let us know which airport it is and I'll check it against my database and if it should work we'll suggest a way to update yours.
😕