When i open training in qb online a error occurs :authentication failed what is reason behind this

Hi, wa95122.

There are times that your browser is full of frequently-accessed page resources which cause QuickBooks Online (QBO) to act strange. You can log in to your QBO account using a private browser to rule out cache-related issues.

Here are the keyboard shortcuts:

Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P

If it works, clear your browser's cache to speed up the loading process of the QBO website and for the system to start fresh. You'll also want to switch to a different browser. 

For future reference, you can check this article to help you get started: Guide On How To Get Started With QBO.

Reply to this thread if you have any other concerns. I'll always be here to help you.