SQLITE Insert with Select Syntax
You are doing an insert into but you only specify one field (sub), then you select 2 fields, just as the error message says.

insert into  data(Sub) -- only one field
select  fullpath, case -- 2 fields
        when fullpath like "%subA%" then "subA" 
        when fullpath like "%subB%" then "subB"
        else "Neither" end
from data


You would either need to add a fullpath field to the data table and do:

insert into  data(fullpath, Sub)
select  fullpath, case 
        when fullpath like "%subA%" then "subA" 
        when fullpath like "%subB%" then "subB"
        else "Neither" end
from data


Or you need to select one column

insert into  data(Sub)
select  case 
        when fullpath like "%subA%" then "subA" 
        when fullpath like "%subB%" then "subB"
        else "Neither" end
from data