System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1050 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 445.87

Blender Version
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-05-31 21:28, hash: rBfcccee0c3372
Worked: (newest version of Blender that worked as expected)

Addon Information
Name: FBX format (4, 21, 2)
Author: Campbell Barton, Bastien Montagne, Jens Restemeier

Short description of error
Fails to export .fbx animation/NLA in this particular situation. cause unknown

Exact steps for others to reproduce the error
Successfully retarget a .BVH to existing armature. Watch as the retarget makes an NLA strip instead of keyframes. Press play, watch the animation work successfully. Try to export as .FBX, see following error messages

FBX export prepare: Wrapping Animations...
Traceback (most recent call last):
  File "K:\Programs\blender-2.90.0x64\2.90\scripts\addons\io_scene_fbx\__init__.py", line 636, in execute
    return export_fbx_bin.save(self, context, **keywords)
  File "K:\Programs\blender-2.90.0x64\2.90\scripts\addons\io_scene_fbx\export_fbx_bin.py", line 3198, in save
    ret = save_single(operator, context.scene, depsgraph, filepath, **kwargs_mod)
  File "K:\Programs\blender-2.90.0x64\2.90\scripts\addons\io_scene_fbx\export_fbx_bin.py", line 3079, in save_single
    scene_data = fbx_data_from_scene(scene, depsgraph, settings)
  File "K:\Programs\blender-2.90.0x64\2.90\scripts\addons\io_scene_fbx\export_fbx_bin.py", line 2458, in fbx_data_from_scene
    animations, animated, frame_start, frame_end = fbx_animations(tmp_scdata)
  File "K:\Programs\blender-2.90.0x64\2.90\scripts\addons\io_scene_fbx\export_fbx_bin.py", line 2093, in fbx_animations
    ob.animation_data.action = None
AttributeError: bpy_struct: attribute "action" from "AnimData" is read-only

On further research, it is probably the same as the following

I have the same problem but and google it and got here, in my case I had renamed an Action and there was an action assigned to the mesh with this name and there were an action with the same name assigned to the armature. It has to be assigned to just one, even though I think it should always be assigned to the armature. On the outliner you can unlink the actions, it solved my problem. I hope this can be usefull to you.