The IRS now requires users to have an account with ID.me to access their online account. ID.me is a service created, maintained, and secured by a private technology provider. 

If you do not have an ID.me account, you must create a new account using Your Online Account at IRS.gov.  

TAS Resources: 
TAS Tax Tip: Verifying your identity to access certain IRS systems
TAS Tax Tip: IRS online systems access and ID verification updates

IRS Resources: 
Your Online Account | Internal Revenue Service (irs.gov)
Secure Access: How to Register for Certain Online Self-Help Tools | Internal Revenue Service (irs.gov)