Did you move your domain from another mail server / mail service to Mailcow, and are there maybe still DNS records for MTA-STS in place that are pointing to the old server / service?
If yes you have two options:
Just delete the DNS records for MTA-STS (Turn off MTA-STS for your domain)
Change the records to point to your Mailcow server
Important: Option 2 requires some manual configuration and setup on your Mailcow server, which is not documented or officially supported by Mailcow. However, Mailcow does support DMARC and DKIM out of the box, and as far as I know, that should be enough for your server to have a good reputation. Google, for example is checking other mail servers for DKIM and MTA-STS, but you should be fine if either one of those is in place, you don’t necessarily need both.
If you still want to set it up, I highly recommend reading Google’s documentation about it, in order to understand how it works: https://support.google.com/a/answer/33786
And here is a blog post on how to set it up on Mailcow: (Sorry only in German) https://schroederdennis.de/tutorial-howto/mailcow-mta-sts-richtig-einrichten-security-mit-tls/
Hope this helps…