Hi everyone..I am working on delamination analysis of rectangular composite sheet using cohesive zone method.While running the intended code,I encountered an error saying-"F is not a recognized BEGIN command,abbreviation or macro." What does it mean..?? And how to resolve the error..?? Given below is a part of my code:

/*creation of cohesive element

LOCAL,11,0,0,0,th/a, , , ,1,1,

TYPE,2

REAL,

ESYS,0

SECNUM,

czmesh,,,11,z,th/a

csys

nsel,s,loc,z,th/a

nsel,r,loc,y,NY((l_nodes*(b_nodes-1))-1)

nlist

cm,cracktip,node

alls,all

finish

/* Applying boundary conditions

alls,all

*ASK,F_val,what is the value of force to be applied at each end node,100

D,1,ALL,,,1,19 

F,19,FX,F_val,,190,19

F,209,FX,F_val/2,,570,19

F,589,FX,F_val,,1140,19

I run the code upto this point and the above mentioned error arises. Please help me resolve this error. Thank you in advance.