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It's the same in your error message. If you use http-01 validation, the client creates a file in /.well-known/acme-challenge, Letsencrypt checks that file.
But there is a redirect to ssl-purchase.wpengine.io.
It this a redirect of your provider?
ssl-purchase.wpengine.io has a valid Letsencrypt certificate:
CN=ssl-purchase.wpengine.io
25.02.2019
26.05.2019
expires in 51 days ssl-purchase.wpengine.io - 1 entry
Is there an option to remove that redirect? But I see, you don't use a control panel.