Using the *OPC SCPI Command to Clear Output Buffer

Hello,

I am having trouble how to use the *OPC SCPI command on my Keysight E3640A PS when performing several read and write commands using the GPIB write vi.  I get the following error at the shown step:

If I press Continue the VI will continue to run and read me back the recorded value, but will give this error every time I try and read a measurement from the supply.  Before this command I am sending a measure current command and then reading the value here for context.  I was told to use the *OPC command, but am not sure how to do so.  I was also told I should clear the output buffer as there may just be too many old/previous commands that are causing the supply to lag out.  Any suggestions?