Please read the documentation and existing content in SCN.
there are at least 2 STO scenarios, intra-company and inter-company.
Intra-company does not use vendors.
If you already compare EKKO records, then you should be able to tell in which other field they are different from each other, eventually in the document type? in the supplying plant?
If you have errors, then you should clearly describe what error you have instead of just saying " facing some error in GTS" , as this info does not really have any detailed information that could help others to help you.
I recommend to read: Asking Good Questions in the SCN Discussion Spaces will help you get Good Answers
Further you should check your notifications, you can find them by clicking the Actions icon on top right, and on the next page the Notifications tab. There might be valuable information for you.