Save for web error - cannot write file/ writing authorization error

Hi there, sorry if the error is not described exactly clearly. I'm using Photoshop in Hungarian, and I translate from there (for you). I want to create a photo montage on a template size 4320x1080. I followed a tutorial on YouTube, tried the same thing on a friends Photoshop, we changed notes, used the same settings. It worked for him, but did not for me. When I try to export the photos, it basically says: The task cannot be completed, because of "writing authorization error".