Being unable to join the server due to a missing asset, be it barricade, vehicle or other is likely due to a mod confliction. Here are the most common causes and fixes for this issue:

Conflicting mod ID's
Some of your mods you already have installed may be conflicting with the mods we have installed on the server. To fix this, close your game and unsubscribe from all of your workshop items. Then open the game and re-join the server. You may have a longer loading time but this is because the server is installing all the correct mods for you.

Server-side issues
Most of the time, these issues are caused because there is an outdated/deleted mod on the server. These issues are typically fixed quickly by staff. This is more likely to be the cause if many players are experiencing the same problem, so ask around on the discord or the forums if any players are experiencing the same. 

Missing or outdated mod files
Sometimes you may have outdated or missing files from a specific mod installed on the server. To fix this, close your game and unsubscribe from all of your workshop items. Then install all the items from this list. 

Incorrect deletion of workshop files
Sometimes, steam does not remove workshop files correctly. You may have to manually find your workshop files and delete them. Before you do this double check you have unsubscribed from the item. Then navigate to C:>program files (x86)> Steam> Steamapps> Workshop > Content> 304930 and manually delete the folders with the unwanted mods inside. (Note: Some file paths may differ depending on where you have steam installed). 

Validation of the game files
Some of your files may be corrupt. With the game closed, right click Unturned from your Steam library and click properties. Go to the local files tab and click Verify Integrity of Game Files. It may be worth unsubscribing from all of your workshop files first. 

Re-installation of the game
Sometimes a re-install does the trick. With the game closed, unsubscribe from all workshop items, then right click Unturned from your Steam library and click uninstall. Once the game has uninstalled, navigate to C:>program files (x86)> Steam> Steamapps> Common  and if there's a folder called Unturned, delete it. Then navigate to C:>program files (x86)> Steam> Steamapps> Workshop > Content> 304930 and delete every folder in there. Then re-install the game and try connecting.

If none of these are the case, feel free to make a topic and staff will attempt to assist you promptly.