How to set a default application to open a file in mac?

In a mac (10.14.6) I want to set the default application to files of type 'err' (i.e. files like test.err or output.err) to be TextEdit.

For that I follow the description given here, and set the default application to be TextEdit.

However, when I double click on that exact same file in the finder, I see the following text box:

So what am I missing? Why does it nor work?

I don't know why the existing two methods are not working - they both should.

Alternatively try RCDefaultApp which as far as I'm aware still works in Catalina. [I can only test up to Mojave.]

I added this manually using the + button & it actually suggested TextEdit itself. I'd suggest also checking 'Override file creator' & see if that might help.