I have a 26 year old Friedrich gas furnace model GHA075NDC that the burner will not ignite. The pilot is lit and I can turn the fan on, but the burner will not ignite. Can anyone help me solve this problem?

Is the gas valve in the furnace turned to ON.

Yes

When you say you can turn the fan on, do you mean you can turn it on manually?

First things first - we should figure out if this is a thermostat problem or furnace problem. Can you jumper together the two wires connected to the thermostat that control the furnace? If the furnace starts, then it is a thermostat problem.

I connected the red and white wires and nothing happened.

If that applies 24 volts across the gas valve terminals and it doesn't open, it is bad.