The stiffness matrix

General Discussion
1

When calculating the spatial error and convergence order, the determinant of the stiffness matrix is non-zero, but the following error and convergence order are obtained

L2error nan H1error nan
L2error nan H1error nan
L2error nan H1error nan
L2error nan H1error nan
L2error nan H1error nan
L2error nan H1error nan
CL2 nan CH1 nan
CL2 nan CH1 nan
CL2 nan CH1 nan
CL2 nan CH1 nan
CL2 nan CH1 nan

why?

2

If you post your code someone might be able to help you.