I have a 2007 Toyota Sienna that is not starting but making this repetitive dan-dan-dan noise when you turn the key to ignition. I'm praying that it's just the battery but the radio and lights seem fine so I hope I don't have any other real issue. Anyone who can get me suggestion, I sure do will appreciate it.

Try to knock in the starter with long iron in the same time another person start the engine if it start renew it if not check the 12 volt on the starter unplug the slim wire connector from the starter and connect
a 12volt tester lamp to the female connector turn the ignition to the start position if the tester light the problem is the starter if not
the problem is in starter relay or ignition switch

Is the immobilizer, if there is no key in the ignition is ok if flashes, if it flashes when the key is in the ignition the key is bad car wont start.

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probably bad key switch make sure not wrong key

If the dealer/mechanic did the service early the light will come on when service is DUE. Go back to him and have it turned off. Search the web someone may have the procedure posted. Usually simple. Google "Toyota service light reset" or "Toyota Maintence light reset" Try this one

1. With the car started (ignition on), set the LCD odometer readout to total mileage (not trip A or B).
2. Turn the car off (ignition key to the OFF position).
3. Push in AND HOLD the button/knob that switches the odometer
4. Turn key to ON position without starting the car. You will notice the maintenance light blink and go out.
5. Release the odometer button/knob.
6. Turn key to OFF position.
7. That's it! The "MAINT REQD" light has been reset to warn you to change your oil in another 5,000 or 7,500 miles or so.

Reset the "MAINT REQD" Light on Toyota Sienna:

1. With the car started (ignition on), set the LCD odometer readout to total mileage (not trip A or B).
2. Turn the car off (ignition key to the OFF position).
3. Push in AND HOLD the button/knob that switches the odometer
4. Turn key to ON position without starting the car. You will notice the maintenance light blink and go out.
5. Release the odometer button/knob.
6. Turn key to OFF position.
7. That's it! The "MAINT REQD" light has been reset to warn you to change your oil in another 5,000 or 7,500 miles or so.

Hope helped with this (remember rated this help) Good luck.

That light means that you are setting the tire pressure guage. there is nothing wrong with the van.

check the battery when you turn the key does it tick a bunch of times if so you need a battery

I would guess that you have a stretched or loose serpentine belt. Common problem.

is it a buzzing noise? If so, it's the starter relay bzzing because the battery is almost dead. Do headlights come on? If no, charge battery, take ot shop to see if battery is dead or not charging due to bad alternator.
Or, bad starter relay

Usualy there is a key hole located near where the passenger air bag off light is. All you do is find the key hole, insert your ignition key and turn to reactivate the air bag. If this doesn't help, i would make a quick phone call to your nearest toyota dealership, and ask for service.
Good luck!