Nope, even with Sage 50 installed the error is the same. I am also getting this error on a clean install of Sage 50 2018.2 and the SDK 2018.2

If you do a search on the machine, can you find the DLL? Is it present at all?

Generally, to troubleshoot assembly load issues, it may be helpful to use the Fusion Log Viewer. See here: docs.microsoft.com/.../fuslogvw-exe-assembly-binding-log-viewer

Fixed now. Have to reference the Sage.Peachtree.API.dll from the original C:\Program Files(x86)\ location, cannot copy the assembly into the project folder and reference locally.

I do have another issue now though with creating invoice line items, if you can take a look at it, it's in the forum posted above this.

Thank you.