During precheck, Data Transmission Service (DTS) verifies that DTS servers can connect to the source and destination databases.
If the Access Method parameter is set to Cloud Enterprise Network (CEN) or Express Connect, VPN Gateway, or Smart Access Gateway, DTS verifies during precheck that the CIDR blocks of DTS servers are added to the security settings of the source and destination databases. If the precheck fails, troubleshoot the issue by checking the CIDR Block of Source Database or CIDR Block of Destination Database column on the Connectivity Check for Express Connect tab. For troubleshooting details, see What do I do if an error is reported when I connect a database instance to DTS over VPN? or What do I do if an error is reported when I connect a database instance to DTS over CEN? For the CIDR blocks of DTS servers, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.