How can you repair a severe leak from the molded plastic exit valve on an above ground plastic swimming pool?

Walgreens sells a product for vinyl repair that works very well when used as directed. it is called POWER POXY.

The valve needs to be replaced. Do not attempt to repair the valve. The patch will not last and could be dangerous people if it were to separate into pieces. If the valve were to break while in operation or while the pump was running the resulting break could drain the pool.