Two potential options:

A. If you would like a solution using a third party app, see https://apps.odoo.com/apps/modules/15.0/web_environment_ribbon/

B. If you would like a solution without a module, create a custom View like this:

< ?xml version="1.0"?>

< data inherit_id="web.layout">

< xpath expr="//body" position="inside">
  < div>
    < span style="width: 400px;top: 55px;left: -100px;font-size: 40px;
                 text-align: center;padding: 10px;line-height: 30px;
                 color: #f0f0f0; transform: rotate(-45deg); position: fixed;
                 box-shadow: 0 0 3px rgba(0, 0, 0, 0.3); background: #D0442C;
                 opacity: 0.6; pointer-events: none; text-transform: uppercase; 
                 z-index:9999">
    TESTING
  < /div>
< /xpath>

/data>

Users will then see this: