Please kindly contact me, one of our user is unable to login Tahnks

Hi there, i-cruz.

Before you can schedule for a call back, let your user login on a private window. It is the best place to identify any browser-related issues. You can share these keyboard shortcuts to open one:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

If they're able to login, they can go back to a regular browser and clear the cache. A piled cache can also be the reason for some browser-traffic concerns. 

They can also switch to a different browser like Google Chrome, Mozilla Firefox, Safari, or Microsoft Edge.

Also, let's ensure that they've entered the correct credentials.

If the same thing happens, they can contact our support to investigate this further.

I've also added this guide about managing users: Add and manage users in QuickBooks Online.

Get in touch with me if ever you need help. Keep safe!