Note: The "No registered connections found" message can also occur if you try to start a migration from the HCX Cloud (Destination) instead of the HCX Connector (Source). For more information, please refer to: Scheduling Migrations from HCX Cloud Manager message "No registered connections found" KB.
VMware HCX
This issue can occur if there is a proxy configured on the HCX Manager port 9443, and there is no exclusion for the remote site pairing.
Add a proxy exclusion for the remote site pairing, for more information, please access: Configure a Proxy Server