To enable TCS for any Counter-Party, Follow below steps:

[A] Go to "Network Module" (1), Search the counter-party (2) for which you wanted to enable TCS. Click on "edit" (3).

[B] After you click on edit, "Manage Network Connection" will open. Scroll down to see "Company Settings". In TCS, there is three option "Purchase, Sales, Both". You can select anyone as per your requirement. 

TCS option will be successfully added for the counter-party.

Keyword: TCS, Network Module, Counter-Party.