Why can't I fit my design again after back annotating the pins?
Sometimes a design can be fitted without pin assignments but fails to be fitted again after back annotation of pin/node assignments by Tools>Back Annotate Constraints in ispLEVER Classic.
The fitting algorithm is an optimization algorithm that its optimization result (that is, the result of the placement and routing) is dependent on the initial conditions. The reason why the design fails to fit again after back annotation is that the initial conditions for the fitter are different from the previous run as the pin and node assignments are a part of the initial conditions.
To resolve this problem, you can back annotate all the pins and nodes, then rerun the design to see if it can be fitted.
If you still see the fitter error, you can read fitter report in detailed format, by checking Detailed check box in Tools > Fitter Report File Format, if it can be generated, to find which Generic Logic Block (GLBs) are more crowded, and manually remove some nodes from the GLBs to other GLBs in Constraint Editor, then rerun the fitter.