Thank you for sending your Spine project! As far as I tried to export your skeleton using Spine 3.8.99 and loaded it with the latest spine-unity 3.8, it can be imported correctly and I can set Skins without any problems. Also, I have confirmed that the active skin placeholder can be switched without problems. However, your project seems to have been created with a slightly older version (Spine 3.8.75), so you might want to update to 3.8.99. It is always safe to update patch versions.
If you export with Spine 3.8.99 and your skeleton still does not display correctly, then there is a problem with your implementation on Unity. Could you send us a minimal Unity project so that we can reproduce your problem?