If you go to Utilities/Disk Utility and select your macOS system HDD anf then right click & choose "Get Info", you will see how is your drive formatted in the field "File System"
i.e : File system : Mac OS Extended (Journaled)
It could be that you are using the "Case Sensitive" option instead of the regular one and if you do, then Caps letters or special characters in your HDD name or in folders could impact the read/write access to them.
It's also worth checking if your up to date with the latest macOS version matching your Mac model.
Latest is High Sierra 10.13.4 (17E202) -> you get the "(17E202)" info by going to : "About This Mac" -> "Overview" and clicking once more on "Version....."