Solver GRG doesn’t take 10 variables

[Zbysek]

Hi, I am working on school project trying to construct equation which describes tabulated data.
So far I’ve gotten to final form of the equation and am applying nonlinear GRG to obtain better coefficients for the equation.
The problem is that Excel does not accept full set of variables that appear in the equation (10 variables a2->d) for the regression process.
I have got to feed it manually "per partes" until I get to acceptable solution. So far got to SSE of approx. 266, but with correctly working solver it would probably got better.
Any idea what is done wrong? Thank you, Excel file is attached, hope it is descriptive enough