Here are a few that might help:

https://gizmodo.com/why-cant-einstein-and-quantum-mechanics-...

https://www.theguardian.com/news/2015/nov/04/relativity-quan...

https://en.wikipedia.org/wiki/Problem_of_time

https://physics.stackexchange.com/questions/387/a-list-of-in...

For a good general source if you want to learn a lot more from a good popular science point of view (but one that takes great care to not accidentally mislead by oversimplification as often happens with popular science) about general relativity and quantum mechanics, try PBS Space Time, especially after Dr. Matt O'Dowd took over as host and main writer.

https://www.youtube.com/c/pbsspacetime

Brian Greene's "The Elegant Universe" does a very good, completely non-technical, job of explaining the conflict between the two theories. It then goes on to explain why String Theory is one possible solution to the problem.

There's also a Nova documentary version if parent would prefer to watch something: https://www.pbs.org/wgbh/nova/series/the-elegant-universe/

I'll add an additional recommendation: Stephen Hawking - The Theory Of Everything

Perfect! Thank you.

I liked Sean Carroll's take on this with Steven Strogatz in the recent episode of Joy of Why.

https://www.quantamagazine.org/where-do-space-time-and-gravi...

tl;dr they don't conflict with each other, they conflict with a third assumption that particles have zero size. GR suggests they would be black holes, and those have mathematical difficulties. String theory is an attempt to model particles of finite size and thus eliminate black holes.