The 1080p export option might not be available unless the project is a 1080p project. To set the project resolution to 1080p, create a totally new project and insert into the timeline a video (not photo) with a known 1080p resolution. You can delete the clip later if you want. Once the project resolution is set at 1080p it cannot be changed. To change it you would need to create a new project. Now, with the 1080p project created insert your other clips and export out. See if you get the 1080 export option.
Another thing that you can try is to delete preferences, as that often will solve export issues. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich