[Calisthenics]

First check the PRIMARY KEY which is `userID`. Ensure it is set as AUTO INCREMENT because when this error is given, it is most likely not.

Since you started learning MySQL, read this: https://en.wikipedia.org/wiki/Database_normalization
Do not try to insert all data into one big table, separate them where possible. A new table for attachments and for what inv_X and invdX columns are used.