Cost–per–thousand impressions (CPM) bidding is only available for:
accounts using U.S. dollars for billing currency
accounts that are using prepay billing
campaigns that target the Display Network
campaigns that target search partner sites
CPM bidding means that you pay based on the number of impressions (times your ads are shown) that you receive on the Google Display Network. You’ll use viewable CPM bidding to make sure you only pay when your ads are able to be seen. Existing CPM bids will be converted to vCPM automatically, but it’s best to update your bids since viewable impressions are potentially more valuable.
Read more here: https://support.google.com/adwords/answer/6310?hl=en