Hello,
Based on the error message, I believe this issue might stem from two reasons:
For some reason, the curve fitting toolbox could have been removed from your MATLAB Path. Try the following steps
>> restoredefaultpath
>> rehash toolboxcache
>> savepath
- You do not have license for the Curve Fitting Toolbox
Please perform a license test for the Curve Fitting Toolbox with the following command
license('test','Curve_Fitting_Toolbox')
If you are license to use the Curve Fitting Toolbox, the output of this command should be 1.