@Sector #7G (6 January, 2017, 11:23 PM UTC):
For future reference, please avoid using all capitals in your posts.
Try reloading the game, maybe the server hadn't received the information from your client, which caused some issues.
If it's missing after the reload, and you still can't build anything to replace it, contact the Help and Support Desk with information including, but not limited to, your coordinates, where you deleted your turret, and screenshots of such.
You should submit a ticket to support so they can look into it. https://support.plarium.com/entd/Tickets/Submit/Category/20Not going to start a new thread for my issue (somewhat related)
I had purchased a level 4 missle turret from the Black Market (750 crystals) back when I only had Level 3 Perimeter researched
But now that I have Level 5 perimeter researched, I can not upgrade the purchased turret.