Autocad 2018 came out in April 2017. ARCHICAD 21 came out in June 2017 so it could not incorporate v2018 DWG/DXF support (at that time of the year the new ARCHICAD version is already in beta phase, just as this year). Hopefully, ARCHICAD 22 will have v2018 DWG/DXF support.
Till then, you can download the free Autodesk DWG Trueview application and convert that v2018 file to v2013 file, which AC21 will be able to read. Please not that it can convert DWG files, but not DXF and DWF files:
https://www.autodesk.com/products/dwg
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2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac27