Are you upgrading from a previous Free version of TextMesh Pro?
Make sure that you ended up with the correct version of TextMesh Pro for the Unity 2017.2. You can verify this by checking the name of the DLL contained in the “TextMesh Pro/Plugins/Editor DLL/…” folder.
The DLL should be named “TextMeshPro-2017.2-1.0.56-Editor.dll”. If that is not the case, then you ran into an Asset Store issue where it gave you the wrong version of the asset.
To get around this issue, simply download the correct version of TextMesh Pro via the direct link available in the top sticky post in this forum.