Skeletal mesh bones cannot be selected in the viewport of the IK Retargeter. They can still be selected via the bone name hierarchy. 
Reproduces in the following:
//UE5/Release-5.1 CL 2390901
//UE5/Release-5.2 CL 24887336
1. Create new Third Person Template in 5.1 or 5.2.
2. Add a new IK Retargeter in the content browser.
3. Set SKM_Manny as the source, and SK_Mannequin as the target.
4. Select Edit Mode on the left side of the IK Retargeter window
Result: 
Skeletal mesh bones cannot be selected in the main viewport
Expected:
The bones can be selected in the main viewport when edit mode is active
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-184901 in the post.
| 0 | 
| Component | UE - Anim - Rigging - Retargeting | 
|---|---|
| Affects Versions | 5.2 | 
| Target Fix | 5.4 | 
| Fix Commit | 29810019 | 
|---|---|
| Main Commit | 29810036 | 
| Created | May 2, 2023 | 
|---|---|
| Resolved | Nov 17, 2023 | 
| Updated | Jan 11, 2024 |