Hi Nguyen,
Cert validation is showing that root cert is not self signed and that is why cert import will fail.
What cert you are using, wildcard or standard?
Make sure you upload the cert chain properly in ".cer" (base-64 encoding) and private key in ".key" format.
Regards,
Syamantak Omer
Syamantak Omer | Community moderator | Sr.Staff TSE | CWNA | CCNA | RASZA | RICXI