We primed a rm for painting ,but now there's cracks, how do we fix it?

Need more info and pictures of what is cracking. Is it the new primer or the wall itself and is your wall wood, drywall, concrete?

If you have cracks after painting, something shifted in your house. Have your foundation checked to see what shifted. Shifting can break water pipes, cause leaks in water and gas lines, and cause cracks that continue to widen. There may be another cause, but there is little sense trying to fill cracks and tape and bed over them, until you get 1-3 free foundation estimates. At least you will know. ☺️

ugh. spackle? I like the very very light fluffy stuff ( ask at the hardware store) . for some small holes and small gaps you can try toothpaste.

Hea259...Is this wood wall an exterior wall? Heat from sun will cause wood to expand and contract when cool. Did you test previous paint if it's an oil base? Make sure you are using quality primer and let it thoroughly dry before second coat. Most primers will fill and cover minor cracks. If it's deep, you need to sand it out or scrape it off or consider texturing (of course, not normally done on wood). Find a good tradesman/painter for advice.