Ok, so the guy I made was working fine until I think I touched something or so. One half of the rig is moving just fine with the mesh, but the other half (The half I duplicated and scaled -1 on the x-axis to get it on the other side) is not moving the mesh with it. I did select the mesh then the armature and CTRL-P with automatic weights, but still something went wrong. The foot on the same side is also being affected by the foot on the first side. Does anyone see a mistake I could have done? .blend file
The issue is that you have skinned your mesh before symmetrizing your rig.
Result: the newly created part of the rig isn't skinned properly to the mesh. And I also guess your mesh may not have vertex groups for those new bones.
First, make sure you have vertex groups with the same names as each deforming bone.
Then, you'll have to edit the weights of your mesh in order to assign the right side properly to the right side bones. Either use weight paint mode, either edit mode via the vertex groups pannel.
I'd try some steps:
0) Alt+P to clear parenting; remove the armature modifier;
1) check and fix the normals of the mesh (Ctrl+N in edit mode);
2) remove all vertex groups of the mesh;
3) check that the right-hand side bones have checked the Deform checkbox in bone properties;
4) when everything is OK, parent the mesh to the armature with automatic weights.