This is already a reported issue, Smpelectricx2 and Mancinid.

Some of our users are unable to create any transactions on the Projects page. This can be the reason why it's not syncing to Tsheets.

While we're waiting for a fix to be rolled out, you can try to create an invoice/esimate outside the Projects page. Let's just ensure to select the customer's project, so it'll be linked to the correct one. Here's how:

1. Click the Gear icon.
2. Select Invoice/Estimate.
3. Fill in the necessary fields.
4. Click Save and close.

I also suggest reaching out to our customer care support, so we'll know how many are affected by the issue. Here's how:

1. Click Help inside QuickBooks Online. 
2. Select Contact us. 
3. Enter "Error when creating an invoice on the project page" in the description field, then click Let's talk. 
4. Select either Start messaging or Get a callback.

The chat line is open from Mondays to Saturdays from 4 am to 9 pm PST. Then, the phone support is open from Mondays to Fridays from 6 am to 5:30 pm PST.

If there's anything else that we can do for you, don't hesitate to drop a comment below.