I assume you left the question mark ? off your last post. Not sure if english is your first language.
But, yes you need to provide a lot of context for especially an error related to something being “un-excepted”.
Ideally, this would include first the .log file also produced by the exporter, in a post. They display really well in forum if you place lines 3 back-quotes, `, on lines just before and after. If it is really long, what you really need to post & LOOK at yourself is the material being worked on when the problem occurred. To reduce post size, you can just include for the last mesh up to the the error, e.g.:
processing begun of mesh: MyMesh_name
processing begun of material: problem_material
Now that the issue is about the material
problem_material, you need to take a screen shot in the
Shader Editor of that material. Here is an example of you I added stuff to for how the principled maps on export. Without some actual context, you are completely on your own.