Defining a representation on $SL_2(\mathbb{Z})$

Question

I want to define a representation on $\text{SL}_2(\mathbb{Z})$, or generally, over any free group with two generators on GAP.

I am using

repsn 

package for this. I have written the code

LoadPackage("repsn");;
f := FreeGroup("x","y");;
G := f / [f.1^2, f.2^3];;
rho := IrreducibleAffordingRepresentation(Irr(G));

But getting error on GAP (SAGE online interface): https://sagecell.sagemath.org/?z=eJzzyU9MCUhMzk5MT9VQKkotKM5T0rS25uVKU7CyVXArSk11L8ovLdBQqlDSUaqESLmDpNIU9BWi0_QM44x0FNL0jOKMQZShFpCpBRLVBTNjgaqLMvJB6j2LilJTSpMzk3JSHdPS8otSMvPSg1ILilKLU_NKEksy8_M0gEo03DWjjWM1rQF-MSwg&lang=gap&interacts=eJyLjgUAARUAuQ==

Could someone please tell me a fix?