Did you add the country of Neapolis (Tag=NEP) into the game since normally it does not exist ?
I tried creating the country of Neapolis inside 00_default.txt and then adding a new family and a ruler to that family that belongs to Neapolis, and it does work.
If you're still having trouble, could you post as attachments your
\\ImperatorRome\game\setup\main\00_default.txt
\\ImperatorRome\game\setup\characters\00_NEP.txt
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EDIT: Here's mine that I just tested, you can search them for the text
## adding family problem
and you'll see what changes I made to the vanilla files.
If you have not added any new characters before this, then the character index of 367 in the 00_NEP.txt file works since it's the next index after the vanilla ones.