Dear Tolga BALCI,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together and try to move in the resolving direction.
I understand your concern and I have tested the behavior with the both existing and new lists in the Microsoft Lists, where I am able to add the new column in the modern experience UI without any issue:

Since you are having the problem in multiple browsers, I also check whether there is any service incident related to your situation but it’s negative as there is no incident related to it.
I would like you to double check whether the same problem happens when you perform the same action in InPrivate or Incognito browser. If yes, then I suggest you raise a support ticket from the Office 365 admin center, where the related team will collect the HAR logs and find out the cause of the problem. Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket.
As a workaround to create a column in the Microsoft List, please follow the below steps:
Open the List in Microsoft Lists> Settings> List Settings> Scroll down to Column> Click on Create Column> Fill the details as per your requirement and click OK.
Thanks for your patience and understanding. Have a good day!!
Best Regards,
Sophia