Problem scenario
You are trying to run a cryptsetup open command. You enter the passphrase, but you get "Requested offset is beyond real size of device." What should you do?
Solution
Try starting over with that partition. Can you create a partition that is at least 16 MB? If it is less than that, or there are some overlapping partitions, that may be your problem.
This posting provides relevant details: https://superuser.com/questions/1557750/why-does-cryptsetup-fail-with-container-10m-in-size
There are sgdisk commands that you can run to view the size of your partition (e.g., sgdisk --print /dev/sda).