You can add a Sketchfab model to any Squarespace page where you can add an Embed Block.
Select the Embed option when you’re inserting a new content block:
Then, copy your model URL from your browser and paste it in the Embed Block field:
Or you could click the small embed icon (</>) and copy/paste the Sketchfab model embed code (iframe) there.
If you're still having trouble, check out the Embed Block Squarespace guide.