Acctivate doesn't see that the lot/serial number had been created yet at the time of the transaction you're seeing the message on.
Sometimes when creating a transaction to move, adjust, or otherwise use a specific lot/serial number, you may received a message like the following:
Lot / Serial number "20210101" not found for Product "PC20".
When this message comes up, Acctivate this means that, at the time the transaction session is being set for, the lot/serial number didn't yet exist in the program or never has. This will often just mean that the transaction needs to be set with a later date.
Example:
Using the Demo company, let's say you received in serial number "20200101" into the "D" warehouse for product "PC20" and that receipt was posted for 01/01/2021. Now it turns out that the serial number is going to be transferred to warehouse "F" so a new transfer session is created. It defaults with today's date and using the lot/serial number field's drop-down will show the serial number you're looking for. Once the serial number on the transfer is set to 20210101, you can attempt to change the date on the transfer to 12/31/2020. However, once you save the order, it will throw the message above.
This will also happen if you set the date before the serial number existed first and then entered the serial number manually. However, a good indicator if doing it this way is that, when using the lot/serial number drop-down selection, the serial number won't be included in the list if it hasn't been created yet at the time of the transaction. In other words, if the lot/serial number doesn't show in the drop-down, you will most likely get error message.
Alternate Cause - Negative On Hand Quantity
A less common reason you might get a message like this would be if the On Hand quantity for a lot/serial number is negative for some reason. Usually something like this can happen when a positive transaction, such as a receipt, was voided and the negative transactions, such as sales, still remain. Acctivate can still see the lot/serial number as missing because the "cost establishing" transaction for it doesn't exist. If you have a number with this problem or think you do, take a look at this article to see how to address it.