It looks like you’ve built a trestle out of construction grade 2 x 4’s.
Is this an indoor project or for outdoors?
Do you plan to paint it?
If you do plan to paint it, I recommend you caulk the cracks for a more finished look.
If you want to avoid this type crack you will need to remove the rounded corners prior to assembly.
You can cut the corners off in a variety of ways but a table saw or jointer would be the easy method.
A hand plane will do it of course but will be a slower method.
If you don't have time to do it right the first time, when will you have time to do it over?