Several issues could be the cause of this. A few typical causes are listed below: 

• Communication cable is unplugged/damaged. 
  
• Drive is in a boot up state. 
  
• Flash card is unplugged. 
  
• Installed hardware revisions are not correct. PG/PC 
  
• Programming device has not been assigned in NetPro. 
  
• Correct communication path, Device/S7ONLINE, has not been selected.
  
• Incorrect communication protocol has been set. Profinet 
  
• IP address on the programming device is not set to the range of the drive or is the same address. 
  
• Using an eWON connection. Profibus 
  
• Baud rate is set incorrectly. 
  
• Node address on the programming device is the same as the drive. 
  
• “PG/PC is the only master on the bus” is/is not set. This is specific for direct connection to a drive or via a routing method.