How do I troubleshoot iTunes failing to add songs to my iCloud Music Library?
The icon you're getting (i.e. a single Cloud with a diagonal slash from top left to bottom right) means your song isn’t eligible to be uploaded to your iCloud library. 

For example, this can happen if the song file is larger than 200 MB, or if it's encoded at 96 Kbps or less.

For more information you can refer to:


iTunes for Mac: Symbols used in iTunes
About iCloud Music Library icons and status


To quote Apple from the second link above:


  This icon appears next items that can't be added to iCloud Music Library. A song might be ineligible if it wasn't purchased from the iTunes Store or was purchased using a different Apple ID, or if the song file is larger than 200 MB, is longer than two hours, or was encoded at 96 Kbps or less.