If the reason is that you have 5.960p in the Per Unit column then the 'p' is
making the whole entry text and you can't do maths on text. If you wish to
keep the entry as it is then with the unit price in B1 try:
=A2*--(LEFT(B2,LEN(B2)-1))
but it would be much more logical if you simply removed the 'p'