If you are using your license in Tally.ERP 9, then you can check TSS validity in the bottom panel in the Licensing and Services section.
If you are using your license in TallyPrime, then you can check the validity from the Manage License screen:
Press F1 (Help) > Settings > License > Manage License, or click the top left corner on the screen below the TallyPrime icon. For example, you can click the Silver icon
If you do not have a valid TSS, then you can purchase one.