1=success, 0=failure, -1=warning

When a -1 is returned the method was a success but something occurred that you should be aware of.  The LastErrorMsg$ property will be set with the warning information. 

Thanks Steve...

I also get in my code return value -1 after nWrite() but LastErrorMsg is empty. What does it mean?

We would need more information about what you situation. 

What version of Sage 100?  What business object are you using?

Once that information is known there will probably be additional questions to help focus in on what is happening.

starting from AP_ManualCheck_bus, adding a line, (for manualCheckDetail), and adding a line to that (for AP_ManualCheckInvDetail). When trying to write the record of InvDetail I get -1.

I'm using Sage 100 Standard 2021 

In this situation you would check for LastErrorMsg$ at the second detail level.

For example:  If 'oHeaderObj' contains the object handle to the header business object you would check oHeaderObj.Lines.Lines.LastErrorMsg$

You need to check the lines object of the lines object.

That's what I did, and like I said, LastErrorMsg is empty.

Since this is a unique scenario with A/P Manual Check having the detail of detail that seemed like a possible answer.

Do you have Job Cost integrated with A/P?

no

I'm reviewing the code to see where a warning could be coming from and Job Cost integration jumped out as a possibility.  

Any other module integrations like Production Management or Fixed Assets?  Any enhancements installed?  Are there credit card transfers being done?