- first one is you wouldn't have selected the current folder in which you have simulation file and the image you have to have insert or mask the subsystem.
- second possibility is that you might have went to the folder but you forgot to click on that folder. In the below image you can see that i have marked a portion in red, it implies that i haven't selected the folder, if i would have selected that folder, the whole path should have displayed in the marked region in the below image. After selecting, try to mask it should work.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/687073/image.jpeg)