I too am unable to send invoices via e-mail after reinstalling Quickbooks Pro 2018 on a Windows 10 system after migrating from Windows 7.

There were no problems on the older system leading me to believe the problem relates to Windows 10.

• Sending to the same gmail account as previously.
• Installed the current Chrome browser.  Not sure if Web Mail uses this or the Edge which is installed with Windows 10.  There is no indication of which browser is being used when the Webmail screen comes up.
• This is a new computer system (exceeds all required specs) with a new Windows 10 installation.

Error message says

Couldn't sign you in

The Browser may not be secure.

Try using a different browser.  If you are already using a supported brower, you can refresh your screen and try again to sign in.

(Image shown below)

Can anyone offer suggestions on how to resolve this.