The specified alternate ID does not comply with the INVENTORY segmented key settings. It might not be possible to use this alternate ID directly in entry forms.

There is even a Feedback Idea to change the way this works: https://feedback.acumatica.com/ideas/ACU-I-1828

Unfortunately, I can't think of a simple way around this. The reason is because of the functionality that allows you to enter an Alternate ID in an Inventory ID field and Acumatica automatically converts the Alternate ID to an Inventory ID once you move out of the field.

Take the screenshot above. In that environment, my INVENTORY Segmented Key is set to a length of 10.

If you enter "MYSKU" in the Inventory ID field on the Sales Orders (SO301000) screen, then move out of the field, Acumatica will automatically convert "MYSKU" to "AACOMPUT01".

However, if you try to enter "TESTING123enenen" in the Inventory ID field on the Sales Orders (SO301000) screen, then move out of the field, you can only enter "TESTING123" because the Inventory ID field can only accept a maximum length of 10. So you'll get this error message.

Note that you can click the ADD STOCK ITEM button and enter "TESTING123enenen" in the Inventory field because the Segmented Key validation doesn't happen in that field.

As a workaround, you can extend the length of the Inventory ID field using the Segmented Keys (CS202000) screen, but you are limited to a Max Length of 30.

Whereas the Alternate ID field on the CROSS-REFERENCE tab of the Stock Items (IN202500) screen has a max length of 50, not 30, so this workaround only works if your Alternate IDs are 30 characters or less.