This error can occur due to a number of reasons. However, the most common one is incorrect folder permissions. Incorrect permissions to a folder can take away your ability to write files on server. This means your web server cannot create or add new files to that particular folder.
To fix this you have to connected with your site ftp, then you need to right click on the wp-content folder and select file permissions. This will bring up file permissions dialog box in your FTP client. It will show you file permissions for owner, group, and public.
You need to enter 755 in the numeric value field. After that, you need to check the box next to ‘Recurse into subdirectories’. Lastly, you need to click on ‘Apply to directories only’ option. Click on the OK button to continue.