Question: Error, invalid input: op expects 1 or 2 arguments, but received 3

restart:with(MultiSeries):with(plots):

parvalues:={C=2.02,H_liq=8.74,R=22.11,R1=0.0006};

G:=R3*k*(1-R1)/(C*Pec_i);

F1:=1/(R*S+1-S);

F:=F1*(1-R*G*S);

F2:=Pec_i*F;

A1:=Theta0/C=(exp(F2*(1-S))-1)*(H_liq+1/(1-exp(-C/k*F2*(S))));

R3values:= [0,-1,-3,-4]; colours:= [red,blue,green,black]; display([seq(plottools[transform]( unapply([k, subs(parvalues, Pec_i = -20, R3=R3values[i], F2)], (k,S))) (implicitplot(subs(parvalues,Theta0=4,R3=R3values[i],Pec_i=-20,A1), k=0..20, S=0..1, colour=colours[i],gridrefine=3)), i=1..4)], labels=[k,'F2']);

p21:=([seq(plottools[transform](unapply([Theta0, subs(parvalues,k=27,R3= -10^(-1),Pec_i=-20,F2)],(Theta0,S))) (implicitplot(subs(parvalues,k=27,R3=-10^(-1),Pec_i=-20,A1),Theta0=0..50,S=0..1,gridrefine=3)), i=1..4)],labels=[Theta0,'F2=Pec_iF(S)']):display(p21);

Q1:=[op([1,1..-2], p21)]: for i from 1 to nops(Q1) do fprintf("c:/temp/implotdata.txt","%f",Q1[i]); fprintf("c:/temp/implotdata.txt","\n"); end do: fclose("c:/temp/implotdata.txt");

Error, invalid input: op expects 1 or 2 arguments, but received 3

Error, (in fprintf) number expected for floating point format