Your Yotta debit card can be used anywhere that MasterCard is accepted. While Yotta doesn't charge any international fees, the merchant or MasterCard may charge conversion or other fees.
There are some countries where you won't be able to use your debit card at ATMs or with merchants, and the card transactions will decline:
Belarus
Burundi
Central African Republic
Cuba
Democratic Republic of Congo
Iran
Iraq
Lebanon
Libya
Nicaragua
Democratic People's Republic of Korea (a.k.a. North Korea)
Somalia
South Sudan
Syria
Ukraine
Venezuela
Yemen
Zimbabwe