Can’t create videos for past two days. Always, sora displays this error:
Unable to generate
Service at capacity, please try again later
Same problem, same message. 
ChatGPT Plus account.
I’ve been running into the same issues as well. Ever since the credit system was dropped. Also a ChatGPT Plus account.
Error Message:
Unable to Generate
Service at capacity, please try again later.
Exactly that is the cause. The servers just can’t handle it anymore. I’m waiting for the news that they are expanding it.
I advise using it when most people are asleep. That’s what I do at least, because not only do you get that annoying notification, but when it goes through, you have to wait a long time to see another error message that it has failed.
Can’t create videos for past 4 days already and now it takes up to 60 minutes before throwing exception at the end 
Simply put, Open AI “screwed the pooch” on this one (as Tom Wolfe so elequently put it in his book The Right Stuff).
Yes, they changed from two to 1 creation at a time.
Yes, I had the same problem. All these slowdowns, bugs and limitations deserve at least a partial refund of the subscription. I paid for smooth and complete use of the tool, and I’m not responsible for excessive use of the services.
Being told I can only generate 1 image or video at a time when my plan VERY CLEARLY states I can generate 2 at a time is actually just unacceptable.
Probably cancelling my plan, I’m tired of being told I’m paying for one thing and finding out I don’t get what I was promised. False advertising imo.