Hi Avinash,
This is not an error. It means that there are several data arrays in the input mtz file and the program needs to know which one you want to use.
You should therefore add the keyword "data_labels='my_labels'", where my_labels represents the string of the data arrays you want to use (f.ex. FOBS, SIGFOBS).
You can find out which labels are in your file with the command:
phenix.mtz.dump my_data.mtz.
Otherwise, you can use the polder GUI and choose the data label from the drop-down menu. That is most likely faster if you have only one calculation.
Best wishes,
Dorothee