Red Hat started to block older versions of the ISOs for the Remote Exam.
Make sure you use the current one. It may be downloaded from red.ht/rhrex
Be aware, the current ISO is not Secure Boot compliant, so it does not boot if Secure Boot is turned ON in BIOS. Also, an attempt to change the BIOS Secure Boot setting cause the system to ask for Recovery code if Bitlocker in Windows is enabled. Better talk to your local IT department, if you are not familiar with those terms and if you are using a company computer/laptop. Red hat does not ask to make the changes, if you decide to do so, it is your responsibility.