First, check git status.
As the OP mentions,
The actual issue was an unresolved merge conflict from the merge, NOT that the stash would cause a merge conflict.
That is where git status would mention that file as being "both modified"
Resolution: Commit the conflicted file.
What you did was to fix the merge conflict (editing the right file, and committing it):
See "How do I fix merge conflicts in Git?"
What the blog post's author did was:
julita@yulys:~/GNOME/baobab/help/C$ git reset --hard origin/mallard-documentation
HEAD is now at ff2e1e2 Add more steps for optional information for scanning.
I.e aborting the current merge completely, allowing the git stash pop to be applied.
See "Aborting a merge in Git".
Sometimes it also happens we are in another branch. For example, i have branch A and made some changes and switched to branch B and made changes and commit them. Now I want to go branch A but forgot to checkout on A and trying to apply the stash. In this case, also it gives same issue. I faced this issue in the above case.