Welcome to the Community. I'm here to ensure that you'll be able to send your sales transactions in batch and get you back to business.
When sending sales receipt in batch action within QuickBooks Online you'll need to make sure that the filtered transactions is Sales Receipt only. This can be the reason why the Send transactions option is grayed out.
To do that:
Click on Sales on the left pane.
Select on All Sales.
Click on Filter.
Under the Type box, select on Sales Receipt.
Select on Batch actions then Send Transactions.
I've also included some screenshots below to guide you with the steps:
If the issue persists after trying these steps, I'd suggest switching to a private browser. This is to determine if the issue is browser related.
Here are the different keyboard shortcuts to open a private browser: