Yeah, the error message isn't exactly useful for such a case. When the sha256sum for the downloaded file doesn't match it simply tells you that the "mount point does not exist".
Also the component you chose is not one available in eopkg. See eopkg lc for a list of valid ones. In this case editor is probably your best bet. I don't think this will necessary lead to a fatal error, but it might (especially with the space in your chosen component name), and will probably cause issues even if the build works.
Tip: you can also use pkgconfig identifiers for build deps. In this case pkgconfig(slang).
eopkg info slang-devel shows this info in the Provides section
(there are some other little issues in the yml, but since you may not want to request this package for inclusion and submit it as a patch I won't go into too much detail right now)
