Hi, dsfoote,
When you open a BIM 360 model from within Revit, the "create new local" box will be grayed out because when working in a cloud model, it's ONLY possible to create a new local (you can't accidentally or on purpose open the central model itself, as you could with file-based worksharing. You hadn't mentioned that this was a BIM 360 Design model, so I didn't know that.
Do you have all the updates for Revit 2018 installed? There's a required security update that must be installed by Monday August 3 in order for you to maintain access to cloud services. Autodesk Transport Layer Security updates require Revit security fix contains very important information that all customers need to be aware of.
Also see
Transport Layer Security (TLS): Updates required to maintain software access.
and
Revit Errors associated with missing Transport Layer Security (TLS) updates
If you've got the updates installed and are still seeing the problem, please attach the following to this discussion (or let me know if you prefer to send them privately and I'll set up a folder for you):
1. A Screencast video showing the steps you are taking. (Screencast is a free Autodesk plugin that works with most of our software. After installing, be sure to close Revit and reopen so you can see the plugin in the ribbon.)
2. a journal file. Here's how to collect it: Please open a new session of Revit, then open the model as you usually do, and recreate the problem. Then go to
C:\Users\<username>\AppData\Local\Autodesk\Revit\Autodesk Revit 2018\Journals (substituting your username where shown) and find the journal.####.txt file with the highest number.
3. your System Information file: to collect this, type msinfo32 into the Windows search box; when the file opens, save it to your desktop.
Thanks, and let me know if you have any questions!
Best,
Judy_S