HSTS Missing From HTTPS Server (RFC 6797) No IIS installed

Hi there I am doing a bit of testing with a product called Nessus and it has picked up the following

HSTS Missing From HTTPS Server (RFC 6797)

The remote web server is not enforcing HSTS, as defined by RFC 6797. HSTS is an optional response header that can be configured on the server to instruct the browser to only communicate via HTTPS.

The remote HTTPS server does not send the HTTP
“Strict-Transport-Security” header.

Output
HTTP/1.1 401 Unauthorized
Connection: close
Server: WASABI/1.1
Content-Length: 73

Configure the remote web server to use HSTS.

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This Server does not have IIS installed on it how can i enforce this server to use HSTS when responding? What file do i need to change?

Any guidance would be great

Thanks