Make sure you created the field correctly. For the time being, just revert all the modifications you've done and  do something as below...

class YourModel(models.Model):

    _inherit = 'your.model'

    qty = fields.Integer(string='Quantity')

Add this field in your xml file.

After you've done this, goto your apps page and choose your module that you've done modifications. You should be in a position to directly click and upgrade your module.

Restart your server and then come to browser and click on upgrade.

Or in the terminal you can give

./odoo-bin -d your_database_name -u your_module_name

to upgrade your module directly from the terminal.