The master admin email is one way to keep the authentication between your two applications communicating successfully.
To update or check your QuickBooks Online master admin email
Log into your QBO account
In the upper right click on the gear for settings
Choose Intuit Account
Update your master admin email address
Now return to your Kickserv account log in as Owner
In the upper right click the settings icon
Choose Manage Users and locate your owner profile
Does your owner email match our QBO master admin email?