Troubleshooting

The model doesn't appear in the preview

Press Find model. A very small model or one far from the origin may be out of view. Center on origin moves it back to the middle.

"This model has no visible mesh"

The model has nothing the tool can show: its parts may be switched off, hidden or scaled to zero in the original file. Fix the original, or choose another model.

Save adjusted prefab is greyed out

The placement isn't confirmed. Press I like this position, orientation and size in step 3. You need to do this again after every change.

The model floats or sinks after resizing

Resizing doesn't set the model on the ground again. In step 3, tick Snap to ground and press Apply selected corrections.

The model doesn't quite touch the floor

Snap to ground uses a box around the model. For unusual shapes, the lowest corner of that box can be a little below the visible part. Fine-tune the height with Position in step 2.

The preview disappeared

The preview closes when you enter Play mode, close the window or Unity reloads its scripts. Load the model again. The values in step 2 are kept, but you need to confirm again before saving.

The new prefab changed when I edited the original

Keep source link was ticked, so the prefab follows the original. To get a fully separate copy, save again with Keep source link unticked.