You should purchase Health Insurance in Türkiye in order to prevent any regulation complications. 

There is a high possibility that the immigration office rejects the health insurance policy you already purchased elsewhere even though it is clearly stated as a ‘worldwide policy’ with full treatments and conditions. According to the current regulations the policy must be issued by a company which has office in Türkiye and the policy should state 'this health insurance is issued in accordance with the regulations announced by law act 10.5.2016 date and 16/2016 published in Republic of Turkey. Generally, the cost of this health insurance policy varies between 40 to 100 Euros. 

(Some countries – Germany, France, Belgium, Netherlands and Austria – have mutual agreements with Türkiye. Students from these countries do not need to get health insurance here. However, they have to provide an official letter from their home country`s insurance services and get it approved at the Social Security Office in Türkiye.
Others are required to get a health insurance. This would cost between 40-100 Euros.