The left side of the model has it’s normals facing towards the opposite direction, you can tell by the darker shading on the model. Faces are one-sided, which is why you can’t see them from both sides. There are two easy ways you can fix this.
First method: Go into edit mode, hit A to select all faces and press Ctrl + N to recalculate the normals. This should ensure that all faces are aimed towards the proper direction. Alternatively, you can press “Recalculate” in the shading tab on the left toolbar to apply the same effect.
The result shuold look something like this.
Second method: Go into edit mode, select only the left faces, press W and pick “Flip Normals”. This will invert the direction of the normals on the faces you have selected.
The result should be the same as the first process.
It can be difficult to know what your model will look like when importing it into studio, as Blender shows both sides of the faces by default. If you want to know what it’ll look like without exporting it first, you can enable “Backface Culling” in the Shading tab on the right toolbar. This removes all the sides of the faces that you won’t be able to see in Studio, and it’s a valuable tool for those who make game assets.
If your mesh has no thickness but you want to be able to see both sides of the model in Studio, you can do this by going into edit mode, selecting all faces, duplicating them by pressing Shift + D and then Right Click, and flipping their normals by pressing W and choosing “Flip Normals”
If both sides look like this you’ve done it successfully.