Hi Abhishek,
In GET_I_LINES, I could see that, they are enabling text lines, based on the Auth Object: CRM_TXT_ID, and this is also based on the Text Object and your Text ID.
In your case, since we don't see any option to choose Text ID, you would have some default Text ID.
So, please make sure that, you have this Authorization object assigned to you with your TextObject/Text ID. Then I hope you would be able to Edit Text Lines.
I hope this helps.
Please let me know if you need any further information.
Thanks,
Sandeep