I have good weights for my upper arm on the left side, as shown, but how can I apply those weights to the upper arm on the right side. I already tried transfer weights and chose destination as the upper arm right, but it didn't work. Please help
I think what you're looking for is mirroring weights, not transfering them.
First, just for the sake of having a clean file, delete the Armature modifiers you're not using. Second, to mirror weights you need two things: that the bones are correctly named, telling Blender which are right bones and which are left bones (see Naming Convention here*), and that you have an active mirror modifier or the X Mirror option turned on in Options, in the T menu while painting weights. If you didn't have the bones named correctly when you created the Armature modifier and created the vertex groups then you need to either rename them manually in the Data menu or delete them and create the Armature modifier again, recreating the vertex groups.
So, explained in other words:
- Name your bones correctly BEFORE creating the Armature modifier (which creates the vertex groups with the same names).
- Have your model with an active Mirror modifier or turn on X Mirror in T menu in Options tab without the Mirror modifier.
*Extract from Blender's Naming Convention documentation linked above:
If you have a bone that has a copy on the other side (a pair), like an arm, give them one of the following separators: Left/right separators can be either the second position (L_calfbone) or last-but-one (calfbone.R)
If there is a lower or upper case L, R, left or right, Blender handles the counter part correctly. See below for a list of valid separators. Pick one and stick to it as close as possible when rigging, it will pay off. (...)
The workflow for your goal requires you do a few extra steps:
Delete half of your model(assuming there's no asymmetries you want to preserve).
Put a new mirror modifier on it. Make sure its above the armature modifier in the modifier stack.
Weight paint onto the remaining half of the model.
Once your done weight-painting, apply the mirror modifier.