Hi,

Adding an alert inside the form will help in this case ? If yes, use alerts in odoo.

Sample:

<div colspan="2" class="alert alert-danger text-center mb-3" role="alert"
    attrs="{'invisible': ['|', ('comment_single_recipient', '=', False), ('recipient_single_valid', '=', True)]}">
    <p class="my-0">Invalid phone numberp>
div>

Thanks