Their problem is that playing games, useless apps, and wasting time on social media is just as easy now on the new fangled phones with as much processing power as many PCs ...
Hence lack of need for a proper computery device.
And being used to doing work on them.
I'm just trying to imagine collating 300 csv files on a mobile device, and using complex formulae to edit some of that data into usable fields all on a phone or tablet.
It's hard enough on a PC with one 32"(4K) and a 27"(HD) screens getting it all in view .. never mind using the final data in graphs.
Back onto Pedros' issue.
I'm thinking some other process may be using that folder, or some of the files in it for him to get access not allowed.
Files appear to be on the PC itself(not the NAS), and mostly likely on another HDD(the 1Tb black going by the screenshot).
the access not allowed would imply you're not allowed access(unlikely considering that the files can be played) .. so the obvious issue is then a process may be using one or more files, or that permissions for your use may be set to read only.
Check the permissions for the entire folder and that you have full access(or at least the write access) allowance.

