@kevinheng21  Do you have any customization on the Shipments → Prepare Invoice action OR in the Invoices screen?

If yes, just try to unpublish the packages and confirm if this issue is caused by the customization or not.

if the issue has not occurred, then As a next step, find out which package is causing it and work on the fix.

Sometime It’s happened and Sometime It’s not happen.

@kevinheng21  Oh okay! 

 I’m assuming there is customization and it is causing this issue intermittently. 

Do I have to check anything beside customization because I test it, it’s working properly in the process. But It happened one or two time a week.

@kevinheng21  Not Sure. We might review the customization once and get it solved if there are any cache-related issues.

Step 1 - Go to the order type of the shipment and get the invoice numbering sequence

Step 2 - Open the invoice  number sequence found in step1 ,  it will notice that last number is  a number which is already a passed number

Step 3 - Go to the invoice and memos screen and get the last invoice number generated  

Step 4 - Go back to the numbering sequence found in the step 2, update the last number in numbering sequence to the last number found in step 3 and try to create the invoice again.