Microsoft has disabled the users to run custom script on personal sites and self-service created sites. So content editors webparts will not be available by default in these sites.
In order to get it enabled please follow below steps.
- Click on Admin link under Office 365
2. Click on SharePoint under ADMIN center.
3. Click on settings under SharePoint admin center
4. Select the below highlighted options and you will get the content editor appear back under Media and Content