If your user has a Chatter Free user license, you will receive an error when applying Simpplr User permission set. You will need to change the user license to "Salesforce Platform", then you should be able to apply Simpplr User permission set.
To change user's license:
1. Login to your Salesfoce instance, go to "Setup" at the top.
2. Go to "Administrative Setup" > "Manage Users" > "Users".
3. Click on the user's name to go to user detail page.
4. Click the "Edit" button.
5. Change the user license to "Salesforce Platform" license and select the "Standard Platform User" profile and click "Save".
6. Apply Simpplr permission set again.