Try...
=IFERROR(C4*(1+C5/C6)^(C6*C7 ),"")
Alternatively, you can use the following formula instead...
=IF(N(C6),C4*(1+C5/C6)^(C6*C7 ),"")
In the first formula, the full calculation is always carried out before determining whether there's an error. Whereas, in the second formula, the calculation only takes place when C6 doesn't equal 0. So, technically, the second formula is more efficient.
Hope this helps!