At the same time, OP seems a layman, and might be coming from things like Microsoft Word, where “Discard all changes” basically means “revert to last save”.
EDIT: After reading the related issues, OP may have also thought that “discard changes” was to uninitialise the repository, as opposed to wiping untracked files.
If the “changes” are all your files, discarding them for me means basically delete my files, you know, the ones you are trying to add.
At the same time, OP seems a layman, and might be coming from things like Microsoft Word, where “Discard all changes” basically means “revert to last save”.
EDIT: After reading the related issues, OP may have also thought that “discard changes” was to uninitialise the repository, as opposed to wiping untracked files.