After purchasing the game i went into the first mission/Tutorial and got to the part when you click and drag, so I clicked and dragged and have yet to have it work.
This issue has occured for users in the past, and i'm trying to find what the solution to it was then.
In the mean time have you tried re-verifying the game's cache? It's possible that steam failed to download all files correctly and missing files could cause this issue.
If you wouldn't mind trying to re-verify files, and hopefully i'll have found the info about last time this issue appeared by the time you check back :)
Think I found it. Taken from a Steam Discussion Thread (Here)
--------------------------------
A very small number of users has reported this issue and we are still
investigating the cause. We have provided a potential workaround below.
if you are suffering from this issue please report it directly to us
with all the information requested. The more reports we have of this the
sooner we will be able to hotfix.
Workarounds
Some users report that forcing DX9 rendering mode addresses this issue.
If re-verfying doesn't work, give that a try next.
Can you take a screenshot of what ever screen you get when you open the properties and i'll try and guide you through it.
Created a new knowledgebase article for this issue: Unable to click game objects / tag tiles
Assuming complete due to lack of customer response.
Created a new knowledgebase article for this issue: Unable to click game objects / tag tiles