¶ No Skin Cluster Found
You forgot to bind a mesh in your group. Check the meshes and if they are bound to the skeleton.
¶ Too many Vertices
Go to blender and decimate your model. Blender 3.3 - Decimate Mesh To Lower Poly Count
¶ Multiple Materials
The best tip to separate materials is to go into F11—face selection mode—and only select the parts you want to be separated with a double click, then assign a new material.
¶ 4+ Influences
"Too many influences" means a vertex is affected by more than 4 bones, which League of Legends doesn't support.
To fix it, first click on the mesh, then press the "Auto fix 4 influences" button in the LoL-Maya shelf — it will automatically limit each vertex to a max of 4 bone weights.