Hi CarolchiFBG,
First of all, thanks for Mchv2.0's advice on customizing master page. Customizing the Berlin.master does make sense in the current scenario. In addition to it, we can customize either the Berlin.master or olso.master (olso.html), based on which option is chosen under Site Settings > Master page > Site Master Page.
We have done some further tests, as a result, we found the berlin.html file can be deleted via the following steps:
1. Select the Berlin.html file in SharePoint Designer.
2. Right click on it, select Rename.
3. Change the file extension to an irrelevant one, for example, rename it as Berlin.htmlx.
4. You would be able to delete it now.
As a side note, you can revert the Berlin.master to its default version according to Mchv2.0’s advice. (In the Master Pages library, right click on Berlin.master -> Version History, select a version to restore)
Please try the steps above and let us know the result.
Thanks,
Evan Zhang