Install Free SSL on Plesk Control Panel
1.Log into Plesk Panel
2. Click Domains in the navigation bar on the left.
3. Click the domain you want
4. Click Hosting Settings 
5.
Go to the Security section and select the following settings:
Enable SSL/TLS support.
Enable the Permanent SEO-safe 301 redirect from HTTP to HTTPS option.
in the Certificate section, select your SSL certificate.
6.Click OK.
7. You will be returned to the domain you selected previously. In the Dashboard tab, select SSL/TLS Certificates.
8. once login ssl/tls certificate you will get
Let's Encrypt
9. Please click on Get it Free 
10. Enter an email address for any notifications/warnings about the certificate to be sent to
Once enter your email address Select the tick-boxes if you want the www and webmail prefixes of the domain to be covered by the certificate.
Once enter your email address and select the Secure the Wildcard domain, please click on Get it free