Since Mixamo has just removed the option for UE4 download, I want to make a quick tutorial on how to get any in-place Mixamo animation working on Epic Skeleton.
Compiled Project Files
First off here is a compiled project showing off one of the issue with Mixamo animation and what it look like when you fixed it.
Dropbox - Error
Epic Skeleton Without Root
Short Intro. Tried making a full tutorial video but took too long to edit.
The main idea is to remove the root and IK bones from the UE4 Manniquin (Epic Skeleton) then upload the NoRoot Epic skeleton to Mixamo for animation. After you have the animations for the NoRoot Epic skeleton, you add back the root node and everything should be compatible with the original UE4 Manniquin (Epic Skeleton).
Step 1) Create a Third Person Blueprint project
Step 2) Export UE4 Manniquin (Epic Skeleton)
Step 3) Open Maya and set World Coordinate to Z-up (Most important step)
Step 4) Import UE4 Manniquin into Maya
Step 5) Unparent the ‘pelvis’ bone from the root (If the mesh get rotate 90 degree after unparenting then your World Coordinate is in Y-up, do step 3)
Step 6) Export the new skeleton as FBX, call it Manniquin_NoRoot (You can call it any name)
Step 7) Upload the Manniquin_NoRoot to Mixamo
Step 8) Choose in-place animation for Manniquin_NoRoot. Only in-place animation will work for this method that I am showing you. Root motion animation require manual retargeting inside MotionBuilder
Step 9) Download animations for Manniquin_NoRoot
Step 10) Import the original UE4 Manniquin with root
Step 11) Import one animation downloaded from Mixamo to original UE4 Manniquin
Step 12) Select the entire skeleton, and export animation. The animation should be now compatible with the Epic Skeleton.
Step 13) Import animation into Unreal Engine 4