The line number should refer to the number of lines from the beginning of the file.

The message may refer to an item code that is not currently in your list in Sage 50.

eg.

<Version>
"12001", "1"
</Version>

<SalInvoice>
"UNIVERSAL CONSTRUCTION 5", , , , , , , , , , ,
"2", ,"A20-300158052","3-1-2015","0", ,"999.00","0.00" <-- This is line 7
"1001","1", "200","200.00"
"1010", "1", "263.5","263.50"
</SalInvoice>

This is what I expected to be the case, but as I mentioned in my description. The problematic invoice was at line ~1800, and the line in the error was 126 — nowhere near. 

I am not sure where it could have gone wrong then.  It's been almost 20 years since I imported files that large and don't have any samples available anymore.  I know we went through some testing sample files that did have errors but I don't remember any errors that couldn't be located quickly enough.  With files that large and if I was experiencing many errors, I might be tempted to create a spreadsheet template with my item codes and drop in the imp file.  Then use the Excel formulas to match the item codes in the imp file to one's in the inventory list. This might not be the best way but it might work.

I figured it out. Each XML element counts as a line. Including <Version>, <Customer>, etc. I'll leave this here and save someone the headache in the future. Thanks for the insight Richard.

Sorry, I thought that was what I showed in the example above.  I included a blank line that also counts.  The file starts with <Version> as the first line down to where I marked line 7.

That is physically line 7 I agree, but what I'm explaining is the import would qualify that as line 2. Version block being line 1. At least, this is how its working on Sage 50 canada 2023.2.

OK thanks.  It seems weird that the line numbering would be the only thing they would change in 18 years of the current programming.  There is no mention of it in the import document.

I just imported the file I posted above into a sample file of Sage 50 2023.2 and it came up with the same message on Line 8 because the item "1001" does not exist in that sample Sage 50 file as an item code.

Weird. Thanks for the troubleshoot.