If a ticket is raised about missing SSL Certs for a microsite, first you need to check the domain on Plesk: Plesk > search for domain:
If there are no SSL certs against the domain, this will be indicated under SSL/TLS certificates (circled above). To rectify, click this.
De-select redirect from http to https and click reissue certificate;
then click here;
This will apply the SSL cert to the domain again.