Have to find out where the issue lies. Is it via EA app (desktop) or via EA account on your web browser? IN my case it's the browser
Like I can't order on my EA account using chrome or edge, it gives some weird error. But when I do it through EA app, it happily goes through and shows in my EA app list of games (can be minutes or hours / I got their command n conquer stuff and jedi games). I preordered during the beta, and it has a BF 6 place holder but its grayed out and says coming soon unlocks in 26 days (but it took days after order to show up in EA app). Then it still has the BF 6 open beta, also grayed and The red " The beta has expired. "
Then after week  games bought in / show on the EA app, shows up in EA account in my browser, BF 6 hasn't showed up there (yet), but the beta finally has. Since purchasing works on EA app, I didn't bother to trouble shoot the browser side of why purchasing is erroring out (its kinda hell sometimes since apps running on browsers aren't as uniform as they used to / should be, and I 'd rather be in BF 2042 getting cosmetic work done) . If you used the browser only, it may not show up until opening day. 
If you are going through EA account on the app, it could be a it could be a problem with your EA client / pp, you can try reinstalling it and if that fails, do a full app removal from Windows (like revo uninstaller or registry clean of the EA app, if you're not sure how to do that, contact your friendly neighborhood IT man / woman), then reinstall it.
Also try diff payment methods. My friend orders online all the time and he likes to use his AMEX card but some places don't like it but if he visa or master cards it, paypal or venmo is listed those work.
Calling EA app technical support if having issues despite a clean install (and if you are using some old OS like Win 7, or old browser like NN or IE, that could be it and you may have to get current) as the customer service people are reading off a script / can only give general advice
 
You can use epic or steam apps to purchase it (you check if epic links with your EA account, steam does, my bf (boyfriend, not battlefield) linked his EA acct with his steam.