We can edit our masterpages via SharePoint Designer. And sometimes, in SharePoint Designer you can see only .master file. .html file is missing. So you cannot edit .master file.
If this comes out, you can check MasterPage Gallery. If you see .html file in there you click edit item and make sure the following are done for the HTML file:
- In the Content Type section, select HTML Master Page
- In the Compatible UI Version(s) section, select 15.
- also, Associated File should be checked.