This problem usually depends on two things:

• files’ and/or folders’ permission are not properly set (see here)
• you are entering your backoffice with the wrong link: make sure you are using the full website’s url with the right protocol. If your link is https://www.mywebsite.com, then you will have to login using https://www.mywebsite.com/backoffice and not (for example) https://mywebsite.com/backoffice (www is missing in this case)