1. He has lifted weighs on the stage.
2. He has lifted weight on the stage.
3. He has lifted a weight on the stage.
4. He is lifting weights on the stage.
5. He is lifting weight on the stage.
6. He is lifting a weight on the stage.
(Which one is correct?)
All but #1 are gramatically correct ways of describing slightly different situations.
In #1, "weighs" is misspelled.
When you say someone is "lifting weight", that does not imply that only one weight is beign lifted.
1-2-3 and 4-5-6 differ in tense. That does not mean one set is correct and the other is not