Thank you for contacting Syncfusion support.
Based on provided information we understood that you are facing an issue when drag and drop the SfAccordion control in Netcore project. We have tried in a new sample and we could see that drag and drop is happening properly.
Please refer the image below.
Can you please follow the below link for adding controls in netcore project?
Also, can you please confirm whether you added the Assemblies from the below location
Location - C:\Program Files (x86)\Syncfusion\Essential Studio\WPF\126.96.36.199\precompiledassembliesetcoreapp3.0
If the issue is still reproducible, can you please provide the exception details with screen shot to check further on our side.