Hey, the texture is not a part of the model, so whenever you export something to another program or viewer, you’ll need to upload the texture separately as well. Here’s how to do that for sketchfab: https://help.sketchfab.com/hc/en-us/articles/202600873-Materials-and-Textures
Only Blender opens .blend files – other programs like Sketchfab or Unity that support the .blend format are actually taking the object and automatically converting them to an FBX behind the scenes, which does not automatically take the packed image textures with them.
