You must insert a valing billing account (use your credit card or a virtual card). Don't worry, you will not pay nothing, never.
After you successfully added your billing account you will get the key.
Insert the key by replace the string [YOUR-KEY] of the installation script with your key.
You must insert the key into Apparence > Theme options > Social and API > Google API key. From September, 2018 Google require to connect the Google Account to a payment method like your credit card, it's mandatory but in most of cases you will not pay nothing, you will pay only if your website have a very large amount of daily visitors.