This is a new problem that quite everyone is experiencing when saving their HTML file after editing using the new Blogger Design.
I thought it was the template that I was using so I change it to the default blogger template but to my surprise, I still got the same alert.
The crazy thing about this is that if you click preview, your changes will show in preview but the changes refuse to be implemented.
The simple and obvious method that occurred to me to solve this is to
1. Copy all the HTML codes and paste it to an elegant code editor like Visual Studio Code after making the required changes using blogger editor and also after previewing it to be sure that the changes are correctly made and there no errors e.g. Error 400
2. After copying, save your code in the code editor on your PC with the required format e.g. .XML
3. Restore your newly saved code by uploading it your blog.
This is a general problem on all blogger template that blogger.com should correct.
Please kindly use the comment box to state your opinion or a better way to solve this problem prior to the time blogger.com will make the necessary correction.