Tar can be used on green wood in limited outdoor craft situations, especially on exposed ends, but it will not make wet wood stable and can trap moisture if used too heavily.
Green wood still contains a lot of moisture. Tar on green wood behaves differently from tar on seasoned timber because the surface may resist penetration and the wood will continue to shrink, move, and check as it dries. A full heavy coat can slow drying unevenly and may encourage problems rather than prevent them.
The most sensible use is often on end grain or ground-contact areas, where the maker wants to slow rapid water loss or reduce wetting. A light coat on peg ends, post tops, tool blanks, rustic poles, and temporary outdoor craft can be useful. Coating every face thickly is rarely the best first move.
Expect movement. As green wood dries, cracks may open through the tarred surface and new bare wood may appear. Those areas can be touched up later. Tar is a maintenance finish, not a way to force fresh timber to behave like seasoned wood.