Can I use a smaller bike inner tube than it supposed to My bike says I need a size 29" inner tube and I got a 26" wheel what’s gonna happen?
That's not going to work, you need an inner tube of the proper size for your wheel rim diameter and tire width.

The tire width should be printed on the tire along with the diameter. Tubes come in various width sizes that support a range of tire widths.

If in doubt, take the bike or wheel to the bike shop and let the staff pick a compatible tube for you.