cabal-version 1.21 is not valid anymore, 1.20 and 1.22 are options, see https://www.haskell.org/cabal/users-guide/file-format-changelog.html

Author: emmanueltouzery

slack-web.cabal

@@ -2,7 +2,7 @@ name: slack-web
 version: 0.2.0.10
 
 build-type: Simple
-cabal-version: 1.21
+cabal-version: 1.20
 
 license: MIT
 license-file: LICENSE.md