Between 2 Sprints and The Sprint Goal

Ok, so you have two questions.

1: When does the PO organize the Product Backlog?
This is one of the most important jobs of the Product Owner. So he/she does this throughout sprints, continuously. The PO involves the dev team for Backlog Refinement, talks to stakeholders etc.
Whenever the Sprint Planning starts, it would be a good thing that the Product Backlog is prioritizes and the user stories are clear to the team. If not, the planning session can be used to further clarify. Result is a plan for the next Sprint, and a Sprint Backlog that is determined by the development team.

2: Is a Sprint Goal required when the Sprint is started?
The Sprint Goal helps clarify what the team stands for in the upcoming sprint. In this particular case as a Scrum Master I would let the Sprint start without a Sprint goal and use the Retrospective to let the team create insight on why to use (or not to use) a Sprint goal.

Generally speaking, when the Sprint Planning Meeting time-box expired, the team starts the developpement of the new increment.

When the Sprint Planning Meeting is poor (not enough details, US not well refined, lack of acceptance criteria, no goal...), you have some good material to inspect and adapt at the Sprint Retrospective.
I observed that poor Sprint Planning Meeting leads to painful Sprint.

Wonderful :)

Thank you a lot.

Jasper, I have one additional small questions about each item.

1) Your answer means that it is better and more benefical for the team, if PO organizes and updates the Product Backlog before the sprint planning meeting as well as updating during the Sprint . ?

2) At that specific case, what do you prefer to do, if you see that starting the spring will cause loss of money, time or etc. that may cause a damage to the project or company? Do you still let them to start the sprint or what is the solution?

Thank you.

In Scrum, a Sprint cannot commence without a Sprint Goal. There is no need to start with a fully planned Sprint Backlog as replanning can and should be done on a daily basis, but a Goal is essential.

Remember that a Product Owner has the authority to cancel a Sprint if it seems that the Sprint Goal is unattainable. Without a Goal in place the PO is not in a position to determine whether or not value is likely to be achieved.

I have worked as a Product Owner as well as a Scrum Master on different teams.

Regarding the Sprint Goal.....

I have worked with a Dev Team (DT) that struggled with this at times. We could still continue the Sprint but I considered this a "Scrum but."

The idea, as a Scrum Master was to reflect this back to the DT quickly. Sometimes, the DT crafted the goal the very next day. Let us also not forget the recent update to the 3 questions, that places a further emphasis on the sprint goal -- this does have value.

If a Dev Team is struggling to craft a goal or ignoring this every Sprint, then there's a deeper issue.

I do agree with Nitin that if the team cannot or will not set a Sprint Goal, there's a deeper issue. This could be they don't have a well defined product. Or they are not willing to commit themselves to the Sprint goal. Or...

Use the retrospective to gain insights on what's going on with the Sprint goal. Or...

You say they're new to Scrum (4 sprints). You could just tell them they need a Sprint goal and teach them why. This will give them better understanding. Tell them to just do it and see what it brings them.

@Tugce: regarding your additional questions:
1: The PO updating the Product Backlog during the Sprint makes for a well ordered product backlog when starting Sprint Planning. :)
2: If there is a Sprint Backlog that the team is willing to commit to for the upcoming sprint and if there is a product you're developing, there is no need for the Sprint goal to be absent.