Form this sounds like they are importing into the GL journal screen? is this correct?

Unusual approach. Normally let AR auto create the GL journals, no importing needed.

If it is importing, consider exporting a successful prior batch from GL journals and look at all of the source codes on that example, replicate that.

But the likely answer - source ledger must be GL, not AR. Because they are importing into GL - not creating from AR

For clarity, the customer exports AR transactions from company B and imports into GL in company A.  Otherwise, yes, AR would normally be linked to GL if it was the same company.

This worked in Sage 2017 and earlier.  The customer finally upgraded, so I don't know about Sage 2018 or 2019.

It may be that the latest version of Sage forces imported GL Batch journal to use source ledger "GL", but the old version let you get away with leaving it as AR.

Suggest you test tweaking the import file. Change the header code to be source ledger GL,. Then you will know what works.

If this works, going forwards if this import isn't too frequent  client can manually update. Otherwise consider automaton of the import with a macro.

And once you know the answer - tell us :) And ask Sage support directly for their advice,

I can confirm that changing all AR-IN, AR-PY, AR-CR, AR-DB to GL-AR in the import file works. This was Sage's advice. I maintain that this is a bug based on an oversight by Sage engineers. There is no net benefit to suppressing this feature.  If anything, it makes it more difficult to reconcile imported transactions in GL with the source AR.

I imported AR batches from one GL to another recently in version 2018, so I just tested in 2020 and it works there too.  The system I tested importing into is on version "2020 (300 Premium) (Build 0)".  

If you have started a batch with GL entries, and THEN try to import AR, it will fail, but it sounds like that's not what you're doing.  So something else is going on here - maybe the product update is causing this problem?

Did the old system have AR activated in the company where the GL is that you're importing into?  If it did does the upgraded version have it activated? 

Did the old system have AR activated in the company where the GL is that you're importing into?  If it did does the upgraded version have it activated?