Error with subdomain with www

[chainazo]

I created a subdomain and I have 2 inconveniences:
_ I don't know why it automatically redirects to the Sin WWW version (when I have my domain with WWW).
_ https://www gives me error in httpstatus.io

What can be happening? Above all, the second inconvenience worries me more.

[lucy24]

Not nearly enough information, I'm afraid.

Your own server, or shared hosting?
Is the canonicalization redirect, if any, in htaccess or the config file?
How is the DNS handled?

When you say with-www do you mean the subdomain is www.sub.example.com, or do you mean that www.example.com IS the subdomain? (The latter is technically true in all cases, but it isn't ordinarily handled separately.)