How to remove #VALUE from a cell

 Hi, 

Please see the below 

=SUM(R[1]C[5]+R[1]C[7]+R[1]C[9]+R[1]C[11]+R[1]C[13]+R[1]C[15]+R[1]C[17]+R[1]C[19]+R[1]C[21]+R[1]C[23]+R[1]C[25]+R[1]C[27])

Its for a holiday form where it is supposed to calculate a person's leave from numerous sheets.  It is working for all other calculations except one.  Any ideas?  I have done the find and replace technique to no avail. 

Many thanks

Lois