Setting up EM client. When I add my account it comes up "Authentication Error . Unable to validate the user."

New account.  I didn’t try to syncronize anything - just tried to add my email address (account).  This comes up each time I try.

Hi Wendy.

Did you check that you have the correct username and password? One way to check that is to login to the web interface for your email with the credentials you supplied when setting up eM Client.

Which email provider are you using?

Yes.  I verified that prior to posting.  I use Rogers.

Go to Menu > Tools > Accounts and make sure you have the correct settings for your email server.

Hi Wendy or to whomever this may concern,

I know that this post is somewhat older, but remained unresolved here,
because of lack of feedback or because the real solution is missing.

Rogers was an affiliate of Yahoo, but is now an affiliate of Verizon Yahoo,
since Verizon acquired Yahoo and AOL.

Even so, under Yahoo you are required to use an App password for Apps like eM Client and others.
Rogers pages are a bit different from the Yahoo pages as shown below.

This is the solution to the problem experienced by the OP .
BTW, this requirement was already in place when the OP wrote this
and still is…(as of the date of this post)