A test:
Ignition ON, Engine OFF, in 4th gear, Push Button. Dash light should go on.
If dash light stays off with the first button push, pull shifter further to right and back, try push button again. If dash light goes on, change 4th gear switch on transmission.
If dash light goes on with first push, leave on, get under car and check wires at solenoid on left side of overdrive. Removing wires off solenoid should make solenoid click with remove and put back on.
Also, be sure transmission fluid is at bottom of fill plug on left side of transmission. Low fluid will not allow overdrive to engage.
If there is power at solenoid because solenoid clicks when dash light goes on, there is a fluid problem inside the overdrive that needs to be fixed if the transmission fluid is full.
Duane