Sides of Mesh are invisible in game.

Please share the package file. I tried to open your model, but apparently it is made on a new version of blender and the interface is displayed incorrectly.

So.. it's very easy: you just need to flip the normals. You can do this by pressing alt+n, then flip. Select the entire object in edit mode before doing this. To understand in the future whether the normals are turned in the right direction, you can use the Backface Culling checkbox in the material settings.

Hi. Your item is flipped on the Y axis, so all of its normals are flipped as well. You'd need to flip them back, but in this case do things differently:

- open the blend, select the backpack, Object - Apply - All transforms to delta;
- File - Export - Wavefront ( obj );
- switch to edit mode, with +, create two new vertex groups. Name them b__Spine1__ and b__Spine2__;
- change Weight to 0.5, select everything with A, select b__Spine1__, click on Assign, select b__Spine2__, click on Assign;
- create a new color attribute, name and set up it like this;
- give it cut 0 and import back into the ring package, recategorise if needed

The only way is to sacrifice something with a bigger space, so either glasses or hats.
If you go the baby carrier path, the item will have no textures in CAS, only in live mode.