your screenshoot says that it can not create a file. Most likely due to not having a permission - you gave permission to the REST API addon, but not to the user.
There are CUBA level permissions that I would recommend you try to set, and I believe it will most likely solve your problem, check this out:
If that doesn’t work, I would try to see if its OS level file permission problem, that is can your CUBA app write to the filesystem at all - it should, but maybe you changed the location of the file storage?
Maybe make a short experiment without REST API to see if its working File Storage - CUBA Platform. Developer’s Manual
If that doesn’t work, maybe you need to authenticate your service https://doc.cuba-platform.com/manual-7.2/system_authentication.html
Basically it says if no user is logged in, so there is no current user, but system needs to do something on its own, possibly as result of schedulet action , you need enclose the code with authentication.begin(); and authentication.end(); - I think however this is not the case here, error message would be different.