Importing a CA signed certificate fails with the following error:
The following warning(s) have been detected: A certificate chain can not be built for the certificate being imported.
Either the root Certificate Authority (CA) certificate or an intermediate certificate is missing from the list of trusted Certificate Authority certificates (Administration > Settings > Certificates > Certificate Authority)
Please import the missing root or intermediate CA certificate so the trust chain can be validated:
Administration > Settings > Certificates > Certificate Authority > Import
For more, please refer to the technical documentation about certificates.