Permissions determine what people can view Slack in issue detail and who is able to create Slack channel in Slack tab.
Private channel is disable by default from version 2.8.8.1
To edit permission for Create Slack Channel and View Slack Tab:
Step 1: Log in to JIRA Server as an administrator
Step 2: Choose Settings button at top right corner 
Step 3: At drop down menu, select Issue
Step 4: At the left hand side, scroll down then choose Permission schemes option
Step 5: Click on Permissions action
Step 6: Scroll down to Create Slack Channel then select Edit link
Step 7: Add Grant Permisson pop-up, add View Slack Tab and Private Channel permission then click on Show more to view all
Step 8: Grant your wish permission then click Grant
Do the same step for other Permission