"An error occurred while Writing/Updating (Your Game Title)]"
This can be the result of a couple different issues. First make sure the location you’re attempting to download to is not Write protected or Read only. If steam can not successfully access the folder to save your game to it this error can happen.
This can also happen when a file Steam needs to access is locked and cannot be opened or edited. This usually happens when the game in question is currently running, or any of its files are opened on another program.
When all else fails, restart steam completely.
For further assistance with the Steam Client, we recommend contacting Valve’s support team Here