Yes @JuM but you cannot change payment methods on Squarespace Commerce with developer mode or code. Squarespace is a website builder not a hosting platform so you do not have access to the backend code like you would on WordPress or Drupal. The only current workaround is to embed an alternative commerce platform within Squarespace, and so I wrote an article about embedding Shopify Lite to take payments with Klarna. As @Bartosz_106commented, this isn't a perfect solution, but until Squarespace add Klarna to the platform, it is one of the only options available today for someone in invested in Squarespace who needs to accept payments this way.
I agree entirely. Unfortunately it can take some time for payment methods to be added so until Squarespace integrate this, you are left within the workaround of using an alternative e-commerce checkout on your Squarespace site, or moving to another platform entirely, like Shopify.
It’s far from ideal, but it is how things stand today.