What is most likely happening is the game (or the Steam Client) is not being run with the proper Windows User Account administrative permissions - which is needed in order for the game to read/write files on your hard drive.
Ensure that your Windows User Account has the proper administrative permissions to run programs and apps. Please ensure that you are running the game in administrator mode if possible. Please visit Microsoft Support's website for information on ensuring administrative permissions: https://support.microsoft.com/contactus/
1) Run the Steam client as an Administrator
- Fully close out of Steam and restart your computer.
- After restarting your computer, right-click on the Steam icon > select "Run as Administrator"
- If that does not work, go to step #2.
2) Run Don't Starve Together as an Administrator
- Run Don't Starve Together dontstarve_steam.exe as an administrator in your game folders (Steam\steamapps\common\Don't Starve Together\bin\dontstarve_steam.exe)
- Right-click on it and select "Run as Administrator"
3) Check your Anti-virus, anti-malware, Firewall, and security settings
- Depending on your firewall, security, and antivirus settings, occasionally it may block a program that you want to access. Ensure that your firewall, security settings, and other antivirus software is allowing the game to do so. You can find more information here: https://support.steampowered.com/kb_article.php?ref=9828-SFLZ-9289
Note: You can also ensure that Don't Starve Together and your Steam client programs have both been added to your security software's or firewall's allowed/trusted programs list, so they are not blocked from being able to read/write your hard drive.
If Steps #1, Step #2 and Step #3 do not help, please try Step #4 and these should be able to help resolve the issue.
4) Check your Controlled Folder Access settings
If you currently have Windows Ransomware Protection enabled, it may block the game from being able to access, read, or write files on the hard drive. If that is the case, you will need to allow/add the game’s .exe file (dontstarve_steam.exe) through Controlled Folder Access, in order for the game to be able to do so.
Here’s how to do this:
1) Select Start > Settings > Update & Security > Windows Security > Virus & threat protection; In Virus & threat protection, scroll down to Ransomware Protection, click on “Manage ransomware protection”
2) Check to see if Controlled folder access is ON, if it is, click on “Allow an App through Controlled folder access”
3) Click on + Add an allowed app > choose “Recently Blocked Apps” if Don't Starve Together was blocked previously or “Browse All Apps” and manually add Don't Starve Together. Each of these options will add dontstarve_steam.exe to the controlled folder access. If you are not sure where the file is located, generally it is within your Steam folders at \steam\steamapps\common\Don't Starve Together\
4) Once added, launch Don't Starve Together once more.
Should the above troubleshooting steps not be of help:
Boot Windows 10 into Safe Mode, and then go into the C:\Program Files (x86)\Steam\SteamApps\common\Don't Starve Together\mods directory (or wherever you installed Don't Starve Together), and delete all the directories/folders starting with "workshop-"
Should that not be of help, you can also try simply not having Steam launch automatically when Windows starts. More info here: http://forums.kleientertainment.com/topic/77321-unable-to-play-at-random/?do=findComment&comment=896900
If you continue to face your issue despite all of the above steps, please submit a bug report in our official Don't Starve Together Bug Tracker.