I bought iPhone in Singapore why I can’t use my phone in Indonesia

It is because your phone is carrier locked in Singapore when you bought it. You need to contact your carrier to unlock your phone.

You can also check this link How to unlock your iPhone for use with a different carrier - Apple Support

You haven't explained why you can't use it...

As suggested above, if it is carrier locked you will have to ask the carrier holding the lock to unlock it.

If you have some other problem, you need to tell us exactly what's wrong.

Ask the carrier to unlock their sim.