Preview of the JPN version of the site.
So, I had this issue (already solved personally) where I get the same "Error" thingy when inputting my Access Code, apparently, I figure out that the ENG version of the site is completely "BORKED", meaning that it's broken.
What's the solution? Use the JPN version of the site...
Go to my-aime.net
scroll to the bottom and find the "日本語" button which should allow you to switch to JPN version of the site.
make sure the site is entirely in JPN, else you gotta redo the 2nd step. (check image for the preview of the site)
once you're in the real JPN version of the site, login with your SEGA ID (assuming you've already made one. If not, try to search for how to create a SEGA ID on google or somewhere, me don't know.) and input your access card.
optional, but you can input the name of the card if you want
once you're in the screen where you were told to input your credit card, congratz, you've done it... the card is finally bind to your SEGA ID...
Finally, go to maimaidx-eng.com
login with your SEGA ID
welcome to MaiMai boeh!
I do hope this helps... It took me like around 2 weeks to figure out why I can't even enter my access code in my-aime (You can refer to my previous post I've made in this subreddit).
The only answer I found recently is DO NOT use the ENG version of my-aime site for the time being.
Enjoy and good luck.