APDL Optimization: There are too many levels of *Do

[netsonicyxf]

The APDL basically models a grid (space truss) structure,
and tries to optimize the section dimention of the truss to get minimum weight while maintaining the proper stress.

When running /OPT in APDL, it shows Error: "There are too many levels of *Do"

The OPT part is as follows

/OPT                         
OPANL,'opt1','lgw',''
OPVAR,D1,DV,0.02,0.07, 
OPVAR,D2,DV,0.03,0.06,
OPVAR,D3,DV,0.01,0.05,
OPVAR,DMAX,SV,0.01,0.07,     
OPVAR,SMAXE,SV,180000000,210000000,
OPSAVE,'OPT1','opt',' '       
OPVAR,WT,OBJ,,,    
OPTYPE,FIRS            
OPFRST,100,100,0.2      
OPEXE          
*STATUS