Well, there is something wrong with www.upfront-rentals.com: it's not sending the required intermediate certificate (but some browsers can deal with that in a few ways).
However, your screenshot is quite weird, as it's not showing the actual Let's Encrypt certificate from the www.upfront-rentals.com website!
So maybe there are 2 things not correct or the screenshot is a red herring and is just due to some antivirus software that you have installed. In any case, the screenshot does not resemble the actual website.
Are you the system operator of www.upfront-rentals.com?
Edit:
Nevermind the screenshot: that's just your browser asking for a client authentication certificate, I'm getting that too. That said, the website is not correctly configured either way.
OpenSSL is also complaining about a too small DH key, so I guess the website is probably terribly configured in total.. Unfortunately currently the SSLLabs server test is not working for me, so I can't put the site to the test.