You have syntax error. Replace [] with ()
And while at it, not good idea to start variables with UpperCase since these are meant for use by Mathematica
Solve[{0.00006*(0.4 + 4*6*0.4^2.5/(0.2^2.5 + 0.4^2.5)) - 11.5*10^-6 == 5*10^-6*(cv + 4*6*cv^2.5/(0.2^2.5 + cv^2.5))}, cv]
{{cv -> 0.06437393881 - 0.1981458609 I}, {cv -> 0.06437393881 + 0.1981458609 I}, {cv -> 223.2363212}, {cv -> 223.2363224}}
