The 3D object you have selected cannot be assigned to the task because it is not a Resource. This is likely true for the whole BIM file. To confirm this, check if the file is missing from the Resources window.
In Synchro, a 3D object cannot be assigned to a task directly. All 3D objects must first be assigned to Resources (Material, Equipment, Location, Human).
To create Resources from the 3D objects,
1. Select the file that contains the 3D object in the 3D Objects window, at the filename node (eg. Structure.dwfx)
2. Right click and select Resources Wizard. This will guide you through the Resource creation process (as you normally would on import).
3. Choose Assign to a new Resource
4. Enter the relevant details, and choose the Resource Type.
5. Choose the bottom option, Add Resources underneath, build tree. This will replicate the 3D tree for the file as the Resource tree, giving you the most flexibility when assigning to tasks.
6. When the 3D objects are assigned to Resources, you will be able to assign them to tasks.