Can I claim subsidy on a used or already bought tractor?

Generally no on both counts. Most government farm machinery subsidies require that you apply and get approval or a permit before you buy, and that you buy a new machine from an approved dealer with a proper bill, so a tractor you already purchased without prior approval usually does not qualify. Used or second hand tractors are normally not eligible for subsidy at all, since the schemes target new machinery. There can be variation between states and schemes, so do not take this as final. Check with your district agriculture office whether any window exists, but plan future purchases by applying for subsidy first and buying after approval.