I appreciate you for trying some troubleshooting steps before posting here, @saedsulub. 

I can sense a browser-related problem, and accessing sites using private mode prevents storing cache and cookies that might cause problems. 

Let your invited user accept the invitation using a private browser. Here's how: 

• Google Chrome: Hold Ctrl + Shift + N 
• Safari: Press Command + Shift + N
• Microsoft Edge or Mozilla Firefox: Hit Ctrl + Shift + P 

If the problem persists, I'd recommend contacting our Customer Care team to help you check what's causing this to happen. They can request to investigate the issue whenever necessary. 

For future reference, check out this article to know more about transferring the role to manage your books: Change your master admin user.

I'd be glad to help you again if you need further assistance in managing your QuickBooks company.