Dear Marc Groslambert,
your problem is the same error if described in one of my posts in the topic
"Bayesian inference with multiple forward model with different numbers of provided measurements but with one common discrepancy model and parameter?
To be able to set c to constant you need to change the file
uq_initialize_uq_inversion.m of UQLab. The file can be found in the sub directory ./modules/uq_analysis/builtin/uq_inversion/ of your UQLab-Directory.
I suggest to start by creating a complete backup of all UQLab files in a new directory that is no sub-directory of your current UQLab-Directory. This allows to check latter, if an error is occurs due your changes of UQLab or if its is
already in the original UQLab-files.
As next step, change to the sub directory ./modules/uq_analysis/builtin/uq_inversion/ of your UQLab-Directory and create a copy of the file uq_initialize_uq_inversion.m denoted by uq_initialize_uq_inversion-org.m (do not use _org but -org such that Matlab will ignore this Backup file.)
Open the file uq_initialize_uq_inversion.m and replace in the lines 89 and 90 (and only in these two lines) ForwardModel by ForwardModelFromOptValue without activating the global replacement option of matlab and check afterwards that ForwardModel in line 392 is unchanged.
After saving the file, your should be able to run the code and the error message above should no longer show up.
Hope that this helps.
Greetings
Olaf