Hi Vinita,
This error message is not about the location.  As it says in the message, you have to get a license key for Modeller, which is obtained from the Modeller developers.

 This is all explained in the documentation, e.g. shown by clicking the Help button on the Model Loops dialog.  See requirement #3 in the documentation:
<https://rbvi.ucsf.edu/chimerax/docs/user/tools/modelloops.html#key>

You would need to enter that license key into the tool as described.  When you say "successfully using Modeller for homology modeling" I don't know if you meant from inside ChimeraX, or just using Modeller directly. If you were doing it from inside ChimeraX, you should have already gotten this license key and entered it into the ChimeraX dialog.  If you were doing it directly with Modeller installed on your own computer, I don't know where the license key is kept.  Probably somewhere in the Modeller installation.

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Resource for Biocomputing, Visualization, and Informatics
Department of Pharmaceutical Chemistry
University of California, San Francisco


> On Nov 17, 2025, at 11:04 PM, Vinita Pant via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
>
> Dear ChimeraX Team,
> I am using ChimeraX (version: 1.10.1 (2025-07-24)) for filling missing residues, for which Chimera uses Modeller in the background. However, I've encountered an issue regarding the executable location of Modeller. Although I am successfully using Modeller for homology modeling, I am receiving a notification in ChimeraX stating that the license key is unavailable. I have attached an image of the error message for your reference.
> I would appreciate your guidance in resolving this matter.
>
> Thanks and Regards
> Vinita Pant
> Doctoral Student (SRF)
> ICAR- Central Institute of Coldwater Fisheries Research, Bhimtal
> Mobile No.: +91 9536940943