As of now, there's an ongoing investigation where some users are unable to create a transaction under the project pages. Rest assured our engineering team is already working to fix this.
In the meantime, I suggest reaching out to our support team. This way, you'll be added to the list of affected users. Once any updates are available, you'll be notified via email. Here's how to contact them.
In your QuickBooks Online (QBO) account:
Click (?) Help icon.
Click Contact us.
Enter a brief discussion of your concern and click Let's talk.