On select filters section the delete button is greyed out

When building a report I can successfully add /delete fields from the manipluate tab however when I move to the select tab the delete button is greyed out and I cannot remove fields once they are chosen.

Please can anyone advise why this might be happening and how I rectify it?

Many thanks

Problem solved

For users reading this thread:
You have to select the field per click to enable Delete button.