I believe you are using the Bivariate platform. Use the Fit Least Squares platform instead. The Inverse Prediction command in this platform still does not work with quadratic models but there is another option. Follow these steps:
- Select Analyze > Fit Model.
- Select response column and click Y.
- Select factor column.
- Click Macros and select Polynomial to Degree.
- Change Emphasis to Minimal Report.
- Click Run.
- Click the red triangle at the top and select Factor Profiling > Profiler.
- Click the red triangle next to Prediction Profiler and select Optimization and Desirability > Desirability Functions.
- Control-click on the Desirability plot at the right end of the top row of plots.
- Change the goal to Match Target. Do NOT change the desirability values otherwise.
- Enter the target response value as the Middle value.
- Enter Low and High values that surround the target. These values are not critical.
- Click OK.
- Click the red triangle next to Prediction Profiler and select Optimization and Desirability > Maximize Desirability.
The inverse prediction appears as the red number below the profiler for the factor.