Blender flows with the .L and .R for Symmetric posing. Two sides needs to be the absolute same as for the names and positions. Here are the steps. Please read carefully.
Delete right or left half of your rig but make sure you have your hips to head bones. The only thing is being deleted is arms, legs, hands, or anything that will need to be mirriored. Be sure you have all of your roll’s that you want to copy aligned either with CTRL.+N by axis or with CTRL.+R manually.
Rename your bones. You absolutely need to name your bones with correct endings! Left side is .L and Right is .R. So, an example would be (Shin.L/Shin.R) or (Hand.L/Hand.R)
Make sure your 3D cursor is to x0 y0 z0. A way to do this is to be in the 3D view port and press SHIFT+C.
Now we need to change the pivot point to the 3D cursor. One way is down at the bottom of the 3D viewport by clicking the icon, that is second from the left of the mode selection, and change it to 3D Cursor. I suggest setting up your constraints/IK’s for arms, legs, and etc. before going to the next step.
Alright, select your armature and go into edit mode. Copy all that is on the left or right with “.L or .R” and scale it on the X by -1.
Deselecting X-Axis mirror. You have constraints on your bones so in order to keep them be sure to deselect X mirror before going to step 7. To get to this you need to go to your Armature Options in your armature’s edit mode. So while in editmode go to your Properties. Press T in the view port to open or close it. You will find you have three tabs; Tools, Options, and Grease Pencil. Click Options. Keep this check mark unchecked before step 7. Cruicial.
Flipping names. I usually press the [SPACE] button ba efore I deselect the selected half. You will get a search bar and all you have to type in is Flip and you will get the Flip Names option. Now flip by X. Your .001 bones will go from (.L.001 to .R) or (.R.001 to .L)
Reselecting your X axis mirror. Not only reselecting your X axis mirror but besure to select the previous side that you copied from. Before step 9:
The Mirrored Bones rolls. Alright now that The X axis is selected and you have the bones that you recently copied from. Now its the fun part. You know that the other side of the rig needs to be rerolled but the quick fix is simple. Press CTRL.+R and press Enter.
Now you have a perfectly mirrored rig. I put these steps as simple as I could put it. Blender’s Naming for bones is different than Maya and 3Dmax but really simple to use after renamed. I hope this helped. Good luck!