If you are unable to access the "Settings - User" menu in Odoo and are getting a validation error, there are a few steps that you can try to reinstall or correct the user module.

First, try restarting the Odoo server and clearing your browser cache. This can often resolve issues with accessing menus or features in Odoo, as it can refresh the connection between the server and the browser, and clear any cached data that may be causing the problem.

If restarting the server and clearing the cache does not resolve the issue, you can try disabling and re-enabling the user module in Odoo. To do this, go to the "Settings" menu and select the "Apps" option. Then, search for the "Users" module in the list of installed apps, and click on the "Action" dropdown menu next to the module. From the dropdown menu, select the "Disable" option, and then click on the "Update Apps List" button. This will disable the user module in Odoo.

Once the user module has been disabled, go back to the "Apps" menu and search for the module again. This time, you should see the "Users" module listed under the "Apps" tab, with a "Not Installed" status. Click on the "Install" button next to the module, and then click on the "Update Apps List" button. This will reinstall the user module in Odoo.

Once the user module has been reinstalled, you should be able to access the "Settings - User" menu again. If you are still getting a validation error, you may need to check the Odoo logs for more information about the issue, and contact the Odoo support team for assistance.