You must have a custom Work Environment loaded.
Try the 'Standard' WE.
This was a new command in 18, so if you have a WE from a previous version you will not see this command unless you add it yourself or load the 'Standard' WE.
You can also "Save As" in the 3D window and change the file type to BIMx (not possible in versions after 18).
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11