- 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.
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