This error happens when you are migrating your module to a newer version, and one of the fields you are declaring in the constructor is referring through a Many2One or Many2Many relation to a field from another model which has been upgraded in this Odoo version, so it doesn’t exist any more as it is declared.
Solution:
Check that all the fields you are referring exist in the actual Odoo version you are trying to migrate to. To make your job easier, you can debug this error by placing a debugger inside your module function that is crashing, where you should be to able to see the object parameters and identify the field that is equivalent to ‘_unknown’.