It was intended to carry Harriers, but they are so old the RTN can no longer get parts to keep them flying. Harriers went out of production 14 years ago.
Well lots of non-production aircraft are still in front line combat duty use. Harriers have been upgraded and improved continuously like most aircraft. But like so many other countries Thailand chooses not to provide budgets for spares, maintenance, running costs and other non-showy but very necessary costs. It's just easier to get a budget for a new shiny thing than it is to get one to repair the old, not so shiny any more thing.
Not just military. Plenty of budget for new roads and railways but damned near nothing to maintain existing ones.