I have a problem saving a project on my Hard Disk

Hi everyone, today i downloaded Adobe Photoshop 2021 on my Macbook Air. I opened a PDF file exported from AUTOCAD and worked on it, but as soon as i try to save it on my pc it gives me an error that goes like "it's impossible to save the file because you don't have writing capability/access" sorry if it's not clear but i'm not english and i'm not sure of the translation, I'll add a photo down here. I also tried to save the file on the cloud and it works, but i must save it on my pc because i need it for the university. Thanks in advance to whomever will answer, have a nice day!