The following error message appear when you try to start or restart ftp service from plesk panel.

Error: Could not complete the operation: defpackagemng failed: The service cannot accept control messages at this time. (Error code 1061) at ControlService ‘ftpsvc’ with code 1

Reason:  ftpsvc (FTP Service) is stuck or is in the stopping status.

Resolution

Find the process with name ftpsvc.exe in Task Manager and kill the process.
Try to start FTP Service once again from plesk Services Management.