What do you get? The formula itself like =mid(etc)?
If so the cell is probably formatted as text, format as general and if
necessary
edit the cell with F2 and the press enter
If you have a lot of these, one way that usually works is to do an edit>replace
and replace = with = (replace equal sign with equal sign) that usually
triggers
what's needed and it is much faster than pressing F2 and enter in all cells
separately