I can't understand, how should I set up the certificate?

NethServer Version: 7.6
Module: certificate

I edited the certificate and now this information appears, but when clicking on certificate I am asked to choose a file, which file is this?

You’ve configured a self-signed certificate, which is the default option. No other action is necessary.

Apart from creating/editing a self-signed certificate, the configuration page allows you to upload a signed certificate (for instance one bought to a Certification Authority), or request a Let’s Encrypt certificate (a free signed certificate from a trusted Certification Authority).

If you haven’t bought a signed certificate most likely you don’t need to use this upload option.

See documentation

…and honestly, at this point, there’s no reason at all to buy a certificate. EV is dead, and OV doesn’t get you anything in the UX compared to DV. Get your free cert from Let’s Encrypt and let it renew automatically forever.

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Thank you all for the response. So messages about the unsafe site will continue to appear and that doesn’t stop the server services from working?

Yes, unless you either (1) start using a cert issued by a trusted CA (like Let’s Encrypt); or (2) create a local CA, trust that CA in all the client systems you’ll be using, and use that CA to issue a new cert for your neth server. If you have a domain and your neth server is accessible from the Internet, (1) is trivially easy using the built-in Let’s Encrypt support. If you have a domain and your neth server is not accessible from the Internet, (1) is still pretty pretty easy using either this method or this one. If you don’t have a domain, consider getting one–they can be free from freenom.com, or even a paid domain is US$15/year or less.

No, an insecure cert won’t stop services from working.