You don't give us a lot of code to work with - just the one line - but I think you have painted yourself into a corner.
What I think is that
is declared as BST*, and
is delcared as Node* - you imply as much in your comments, so you can't assign a
because a Node is not a BST.
If you are designing a binary tree, then normally the nodes such as Root, Next, and Previous are declared as BST* not as separate a separate class - otherwise you can't treat them as BST's when you get there.
I can't give you concrete code to fix it - I can't read your HDD or see your screen - but look at how you have defined things, and see if you can spot the logical problem!