Import/Exporting certain columns of a table across two databases and excluding duplicate emails in mysql
Toss the db1.tblclients. in the list of columns; it is totally redundant.  And a syntax error.

A suggestion:  Make the SELECT a LEFT JOIN ... WHERE id IS NULL.

And, again, db2.tblclients. is redundant.  (But not harmful.)

(If those suggestions are not enough, show us what you have; there may be more to fix.)