I have been trying to add the transformation spell to my home brew monster but it won't add. I'm using the right tool tips but it still won't work. Is this some kind of bug? 

You mean Tenser's Transformation right?

Have you bought this spell from Xanathar's Guide to Everything, because if you haven't you cannot add it to homebrew.

I bought the entire Xanathar's Guide

Quote from benthorson >>

I bought the entire Xanathar's Guide

 
And you're using the right spell name: Tenser's Transformation ? (not just Transformation, you're including the apostrophe ', etc)

EDIT

I tested this and got it working fine and dandy. So this seems more user error.

Check for typos in the tags: (spell)Tenser's Transformation(/spell)

Obvs changing ( for [ and ) for ]. Should get Tenser's Transformation

What's probably happening is that the apostrophe in Tenser's is getting converted to HTML so instead of “Tenser's Transformation” the tooltip is reading it as “Tenser’s Transformation.”

At the top of the field you can click a little grey button with this symbol:

</> 

That will let you see the code and if you make it an apostrophe there it works correctly.