求助：There are no active degrees of freedom——如何解决?

想做一个结构在大位移静力平衡下的预应力模态分析，命令流如下：
!
! 首先进行大位移静力分析
!
FINISH
ALLSEL
/SOLU
ANTYPE,STATIC ! Static analysis
NLGEOM,ON ! Large deflection analysis
PSTRES,ON ! Flag to calculate the prestress matrix
ALLSEL ! Select all entities.
time,1 ! 设定求解步数.
autots,off ! Do not use automatic time stepping.
nsubst,10 ! Specifies the number of substeps to be taken this load step
outres,all ! All solution items data are written to the database
SOLVE
FINISH
!
! 大位移预应力模态分析
!
/SOLU
ANTYPE,MODAL ! Modal analysis
PSTRES,ON ! Prestress effects ON
MODOPT,LANB,20,0,1 ! 选择Block Lanczos方法,求前20阶模态, 频率范围0~1
MXPAND,30,0,1 ! 扩展前30阶模态, 频率范围0~1
PSOLVE,EIGLANB ! Calculate the eigenvalues and eigenvectors.用Block Lanczos方法求解.
!
结果是大位移静力求解顺利结束，但当执行到：PSOLVE,EIGLANB 时，却弹出错误：There are no active degrees of freedom.
请哪位大侠指点一下该如何解决。多谢！