Hi @Alfonso , pls allow me to sum up here to end this case.
My guess is the account header is defined in the _Layout page, but per the test by Op, he can't find a layout fine in his project as he didn't select all needed options when integrate identity module.
So the next step or the solution is adding the missing file by copying them from a newly created test project. Then can modify that html snippet.
Thanks for Alfonso's effort and reply : )