help with blender and horizon needed

In Blender use the 'Export' option under 'File' and choose FBX. Make sure 'Selected Objects' is ticked.

If you have materials, which I am sure you do, make sure to follow the correct process, as this bit can be a bit confusing - https://developers.meta.com/horizon-worlds/learn/documentation/custom-model-import/creating-custom-models-for-horizon-worlds/materials-guidance-and-reference-for-custom-models

As a basic test, with the default cube in Blender, create a material called 'Base' and make sure it is 'applied' to the mesh. Go to Texture Paint > Image > New and then save the Image as 'Base_BR.png'. The _BR part is important and can cause errors if incorrect, which I suspect is what is causing your import issues, so make sure you read the link above so it can tell you all the tricks. 

Finally, export the default cube in Blender as 'Default_Cube.fbx' and make sure both files are in the same place so you can select them together.

When you import into MHW, shift select 'Default_Cube.fbx' and 'Base_BR.png'. You should then have your 3D model and texture in MHW. 

If all this works then it should give you an understanding of the export / import process and we can then look into your more complex workflow using SketchFab.