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the res.users model has specific security restrictions to prevent unauthorized changes to user records. Certain fields, like emails and phone numbers, may be editable by default for users with fewer permissions, but adding a custom field can cause permission issues because it doesn't fall under the predefined access rules for these fields.

To allow users with limited permissions to update your custom field on the res.users model without encountering the error, you can add an access rule for that field or adjust the write permissions for the res.users model through a few different methods.

Add sudo() for Field Updates: If only users with higher access should modify the custom field, you can override the write method on res.users and use sudo() to elevate privileges when saving the custom field also you can use Custom Write Method to Handle Permissionsand  Override the write method and allow specific updates based on field names.