How can I photograph frescoes in a dim church using available light?

If you want to stay with available light, the main solution is a tripod and long exposures. Keep ISO as low as practical for best image quality, and stop down enough to get the fresco surface sharp, but be aware that smaller apertures will make exposures even longer.

Long exposures work well if nothing in the scene is moving. Watch for people, shifting shadows, or changing light from windows or fixtures during the exposure. You may also be able to use reflectors or diffusers to soften shadows and tame bright highlights without setting up full studio lighting.

If the ambient light is too dim, causes strong color casts, or creates reflections, then you may need to add light. That does not require a studio setup: one or two speedlights can be enough and don’t need mains power.

For the scaffolding, if it cannot be removed, one option is to try shooting with a wider aperture and focus on the fresco so nearby bars blur as much as possible. Another practical workaround is to make multiple exposures from slightly different positions and combine the unobstructed areas later.

Also look into techniques for controlling reflections on artwork, including the possible use of filters where appropriate.