When a user tries to install SMS Magic workflow in sandbox the following error may occur.

Package SMS Magic Workflow Install Failed- "Error Message: The post install script failed"

This issue occurs when user is not having MASS EMAIL permission. This leads to failing the Post installation script and opposing the package from installation.

Solution steps:

  1.  Go to Quick Find/search in salesforce.
  2.  Email Administration->Deliverability->Change access level to All email.
  3.  then try to install the package.  

The issue will be resolved.