The most common reason "Include skin weight" is greyed out, is: more than 110 vertex groups per mesh Object. I know, the armature has 159 bones. If you need more than 110 vertex group, you'll have to split up the mesh.
Other limitations:
No more than 8 material faces per mesh
No more than 21844 tris per material
Other requirements:
(Blender) Apply scale, rotation, transforms to all objects, as well as the armature, before attempting an export > import
If using custom LOD's, their materials name must match. In Blender, that means sharing materials across the LOD meshes.
Be careful about naming both LOD and materials. There are blogs out there on this, I don't think all my notes are up-to-date. Avoiding spaces and special characters is good practice in any event.
There can be other causes for rigged mesh upload failure and we'll need more information to help you.
Which 3D software are you using? Which version? I can help you with Blender issues if you're using the most recent versions but it's been over a decade since I've used Maya or 3dsMax.
Are you using Avastar? Which version? Or another armature to export? Avastar for Blender 2.8 and after is still in beta, as far as I know. Questions should go to their Discord forum.
I have been, with a great deal of help from Ai Austin, documenting some of this at the RuthAndRoth github Reference repo.
https://github.com/RuthAndRoth/Reference. In the readme files, the Wiki, and in the ods spreadsheet. The armatures are free to append or download.
I'm also co-teaching with Kayaker Magic, at Kayaker's Blender Bytes classes - Wednesdays 10am grid time at the Discovery grid. IM one of us for a landmark. We've had one class on exporting rigged mesh from Blender into opensim grids without Avastar, and can do more if people want them.
Good luck! Keep asking! There are so many ways rigged mesh can bork. Several of us in the Kitely Community can help.
