If your phone won't accept the eSIM, one of a handful of things is usually behind it. Work through these in order and you'll clear most cases without contacting anyone.
Check your wifi
Your phone downloads the eSIM over the internet, and your Flex Mobile data isn't live yet. Make sure you're on wifi and that the connection is working, then try again.
Check the QR code hasn't already been used
An eSIM QR code works once. If you've already installed it on another phone, or an install partly completed, the code won't work again. Issue a fresh one yourself: sign in at portal.getflexmobile.com, open the Subscriptions tab, select the line, then select Order under Order new SIM card.
Check your phone is unlocked
A phone still locked to another carrier will reject the profile. On iPhone, open Settings, tap General, tap About, and check that Carrier Lock reads No SIM restrictions.
Check your software version
iPhones need iOS 14.1 or newer. Open Settings, tap General, tap Software Update, and install anything pending. Restart the phone and try the install again.
Make room for the eSIM
Phones can only hold a limited number of eSIM profiles. If yours is full, delete one you no longer use, then try again. On iPhone this is under Settings, then Cellular. Only delete a plan you're sure you don't need, because removing it ends that service on the phone.
Try entering the details by hand
If the camera can't read the code, use the manual entry method with the SM-DP+ address and activation code instead.
If none of that works
Contact Flex Mobile support with your account email and your phone's model and software version. They can check whether the eSIM was issued correctly and reissue it if needed.