Cost-per-thousand impressions (CPM) bidding is only available for:
- accounts that are using prepay billing
- campaigns that target the Google Display Network
- accounts using U.S. Dollars for billing currency
- campaigns that target search partner sites
Explanation: 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.
Read more here: https://support.google.com/adwords/answer/6310?hl=en