First, check if the file or folder is shared with you

1. If it’s a file, open it and go to File > Share > Advanced
2. If it’s a folder, open it and select the Share button in the top menu
3. From the list, make sure you are listed as having access to the file or folder

If you’re still unable to access the file or folder, then it may be the owner of the file or folder needs to grant you additional permission. Your administrator can help you with this.