If you tried to send your certificate and private key and received the following error, you would need to double-check the files.

Correct Key File

When you generate the Certificate Request, a Private Key is also created. You send the Certificate Request to your provider and keep the key generated. When the provider gives you the certificate back, you upload this file and the original private key.

Correct Concatenation

Check that your certificate is correctly concatenated. They need to be in the following order:

-----BEGIN CERTIFICATE-----
(Your Primary SSL certificate)
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
(Your Intermediate certificate)
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
(Your Root certificate)
-----END CERTIFICATE-----

Blank Lines

Check for blank lines between each certificate in the bundle file. This may cause an invalid syntax, and the certificate won't validate.

DNS Settings

Double check your DNS settings. For more details on how to set up your DNS records check this article