I have a gas oven. I frequently bake pizza in the oven but lately whenever I turn on the upper grill, it turns off after few seconds. It gets on when.

A gas oven needs fresh oxygenated air going in and combustion products going out. If either of these ports is blocked, the flame might go out. It also needs some kind of thermocouple to detect that the flame is lit and it's not filling your kitchen with flammable gas. If the temperature detection part breaks, it'll shut off the gas.

This is a useful question and someone gave a useful answer.

I miss having a gas stove

Natural or propane? In either case, check any valves, check for blockages and damage on both the pipes in and the vents out.

Is the gas piped in or do you have a re-fillable gas bottle? If from a bottle perhaps it needs topping off to get the gas pressure up.

It’s a little fan up top in the back hard to see but just goggle you stove and you will see it

A Florida oven?

Sylvia shows up

Don't understand this

Sounds like a bad thermocouple.