The exact error message will be similar to this:

HSCLA2B7 The management console was unable to find a valid mover service partition (MSP) on managed system Server-9119-FHB-SN12345AB.


MSPs (Normally the VIO Servers) must exist on both the source and destination managed systems, and they must be able to communicate with each other to be considered capable of performing an active partition migration. Verify your migration setup and communications between MSPs on both source and destination managed systems.

Check the output from these commands on the MSP(VIOS):
These commands should return a list of known HMC's and connection details.
For HMC v7 - lsrsrc IBM.MCP
For HMC v6 - lsrsrc IBM.ManagementServer

Check the output from this command on the HMC.
lspartition -dlpar

Sometimes you need to reset rsct and rsct_rm on the MSP(VIOS):
stopsrc -g rsct_rm; stopsrc -g rsct; sleep 2; startsrc -g rsct