Google will not allow you to start advertising using Google Shopping until you verify and claim your webstore's URL.
Current Merchant Center Layout
Log into your Google Merchant Center account.
Click Settings - the wrench icon towards the top right.
Select Business Information
Select the Website tab at the top.
Follow the instructions (HTML or Alternate method).
Click Verify & Claim.
You will also want to complete the About your business section.
Here are some help articles depending on which platform your webstore is on:
Old Merchant Center Layout
Log into your Google Merchant Center account.
Click Business Information.
Click Website
Follow the instructions (HTML or Alternate method).
Click Verify & Claim.
You will also want to complete the About your business section.