I had same issue. The cause of mine was my ad blockers were not set to letting quickbooks be a trusted site. 

If you go to the gear icon on your ad blocker find trusted sites and put in app.qbo.intuit.com as a trusted site. 

If you have more than 1 blocker you will have to do this for each one.  This cleared up my problem never have had another issue with this.

Several attempts have proven fruitless to access my account. the system won't accept 6 digits. WTH?

To all quickbook users and emps.

Disabling two/multi-factor authentication does not work. No matter how many times I have disabled it and how many "techs" tried to walk me through it. It consistently re-engages.

It appears this feature is for governmental overreach to backdoor itself into your data. Isn't it bad enough we have an entity that has entitled itself to your hard work? You exchange the skills and labor to provide for yourself and family. Have to keep track of how much time you spend working, and how much money the fed gets to take from you. If you don't give the right amount or are late paying the extortion fees get charged more. Congress can pass laws the irs cannot. I don't recall any amendment that grants the irs exclusive authority to collect fines and fees which would be taxation without representation.

Had same issue. Mine was because I had not listed quickbooks as a trusted site on my ad blockers. Try going into ad blockers gear icon or list of trusted sites and install app.qbo.intuit.com it worked for me.