Welcome to Manager and the forum!
Can you describe the steps you are taking? Note that once you’ve created billable time, to invoice it you need to go to the Customers tab, not the Billable Time tab. If there are customers with uninvoiced billable time, you should see a column called “Uninvoiced” just to the left of the “Accounts receivable” column. Can you confirm that you see this? Then click on the amount showing in the Uninvoiced column, check the boxes next to all the billable time entries for that customer that you wish to invoice, and click the “New Sales Invoice” button.
If you cannot follow the steps as described above, please share screen shots showing what you see instead.