Excel thinks you want to start a formula with the negative sign.  

You can either prefix your entry with an apostrophe or preformat the cell as
text.

If you select the cell after doing the above and look a the formula bar, you
will see Excel tried to "help" you out when it saw the leading minus sign
(you probably thought of it as a dash when you typed it) and added the equal
sign attempting to make it a formula for you. Of course, it is not a valid
formula (no functions are named 've'); hence, the error. You can place an
apostrophe (the unshifted quote mark key) first and then type the '-ve root'
after it to force the entry to be text. Alternately, you can format the
cell, cells of column as Text and then you would not need to use the
apostrophe.

Rick