This was a new problem that I had not come across before on SOLIDWORKS. I was unable to select the “Flatten” icon due it being greyed out.
However I was still able to flatten the sheet metal part by un-suppressing the “Flat-Pattern1” feature in my feature tree.
The reason for the “Flatten” icon being greyed out was because the sheet metal part had a “Flat Pattern” annotation view and this annotation view was active.
By selecting another annotation view, the “Flatten” icon then becomes available again.