I tried to parent bones with automatic weights and 1 part of my model is working properly(the green one), while other is kinda "broken"(the red one) when I try to move bones. I don't know if that's relevant, but the "broken" arm is actually a clone of the original arm. Could anyone help me fix this pls? I could add more detailed pictures if needed.
When Blender automatically generated the weight for the bones it must have not completely covered the area of the bone. What you need to do is select the mesh and go into weight paint mode. Select the vertex group for the bone that is having problems, set the weight to 1.000, and paint the area around the bone. That should fix your problem. Painting may be needed on more bones than just one.