Partner setups can be difficult for us to troubleshoot on the community, so I checked and @cf_evan had a look and was able to provide a lot of useful info.
It looks like your domain currently has CAA records that limit the certificate authorities able to issue certs for your domain.
Most of Cloudflare’s Universal Certificates are issued by DigiCert. You will likely need to either add them to your CAA record or remove that record. You may want to pass this information back to Hostinger if you are unsure how to do this yourself.