Rm: cannot remove 'backup': Is a directory rm: cannot remove 'transfer': Is a directory

rm: cannot remove ‘backup’: Is a directory
rm: cannot remove ‘transfer’: Is a directory

why can’t I remove them?

You need to use other flags with the rm command when you are deleting a empty directory or a non-empty directory. Google your query and you will get a lot of results.
Or read documentation of rm command using

man rm

.

Thanks man
It worked