It looks like MATLAB cannot find the file you mentioned. Although the file exists, MATLAB cannot find it if the file is not in the known path.
Can you load the file from the MATLAB command line?
DOMnode = xmlread('D:\Program Files\Polyspace\R2020b\slprj\_fmu\8ae35f333694fdbb35d722538d0fc9e8\cm_FMU\modelDescription.xml')
If it works, then it means the file exist but MATLAB does not know where it is. So you should tell MATLAB that when asked to find files, look into that folder as well.
To add the path to that xml file to let MATLAB to be able to locate it, you can do the followings:
oldpath = addpath('D:\Program Files\Polyspace\R2020b\slprj\_fmu\8ae35f333694fdbb35d722538d0fc9e8\cm_FMU');
If there are other files in other subfolders in the same project (I assume slprj stands for sl project), you can do the following to add all of the subfolders under D:\Program Files\Polyspace\R2020b\slprj by doing the followings:
oldpath = addpath(genpath('D:\Program Files\Polyspace\R2020b\slprj'));
Place addpath before the loading occurs. It will fix the problem.
Note: addpath is effective only in that session. If you turn off MATLAB and launch it again, you have to add it again.