Hi, 

    While inheriting the default class in your custom module, you should not use _name , use _inherit instead.

Since you have already used it in your code , remove _name = "account.invoice" and try again.

And make sure that in your manifest file in custom module you have added the account module in depends.

Please check the documentation here for inheriting, https://www.odoo.com/documentation/12.0/howtos/backend.html#inheritance

Hope it helps,

Thanks