You cannot open project files from earlier versions directly in ARCHICAD 20. Please use ARCHICAD 8.1 or later version to convert them to a usable file format.
We strongly recommend that you create an archive file (.pla) of each project you worked on with older ARCHICAD versions.
See also the Migration Guide in ARCHICAD Help.
ARCHICAD 20 can back-save project files in ARCHICAD 19 format only (among previous ARCHICAD file formats).