Hi there, @RichJ.
Thanks for sharing what you've done so far to fix the error you're getting when changing from Serial to Lot Numbers. I'm here to help sort this out.
Since you already tried to find all the transactions and deleted the serial numbers, let's double-check if there are still other transactions that have multiple serial numbers. To do this, let's go ahead and pull up the Transaction Detail by Account.
Here's how:
1. Go to Reports menu at the top.
2. Select Account & Taxes on the drop-down, then Transaction Detail by Account.
3. Click Customize Report on the upper left.
4. On the Columns search box, type Serial Number then put a check mark.
5. Click OK.
6. Make sure to select All on the Dates drop-down, then look for the transactions that have multiple serial numbers.
7. Double click the transactions one-by-one to edit.
Once done, go back to the Preferences menu to change from Serial to Lot numbers.
Here's how:
1. Go to Edit menu at the top.
2. Select Preferences on the drop-down.
3. Choose Items & Inventory on the left panel.
4. Click the Company Preferences tab, then select Advanced Inventory Settings.
5. On the Serial/Lot Numbers tab, put a check mark on Enable Lot or Serial Numbers.
6. Choose Lot Numbers, then click OK.
That should get you fixed right up! But if you're still receiving the same error message, please let me know so we can try other steps to get this working for you. I want to make sure that this is fixed.
