Hi, my name is Neil, and I'm an Outlook user like you.
I'll be happy to help you with your question.
The picture of the box that you have attached is asking you for the last 4 digits of the phone number that is attached to your account as a security backup.
So, before the security code can be sent, you need to enter the last 4 digits of your phone number and then click the "Next" button.
So if your phone number was 01234 567899 you would need to enter 7899 as these are the last 4 digits of your phone number.
Once you have entered these last 4 digits, and Microsoft can confirm they match the last 4 digits of the phone number on your account, a security code will be sent to that phone number.
You can then enter that security code in the next step.
I hope this explains the process for you and that you can now successfully enter your account.
Please do let me know how you get on.
Kindest regards,
Neil