“I am a beginner and meditate sitting on a chair.
My main problem during meditation is keeping spine straight.
I wonder if it’s OK to use pillow to support the back?”
Yes, you can support your back with a pillow when meditating.
You can try meditating lying down and may find this a more comfortable way of meditating.
Meditating lying down allows you to keep your spine straight and completely relax all the muscles in your body.
This helps you focus on meditating and many people find it helps them get more from their meditation.
Edwin Leo 