Hi @perjumaryus as mentioned in the error you first need to setup the rates and transit time and then shopify will calculate the estimated time automatically on the checkout page.
Thank you for your help, but still you mention the same thing what is not possible for me to set: transit time, which is not available for the flat rate.
I found on the link you shared the following:
Transit time is available to fulfillment locations in the United States, Canada, France, Germany, Italy, Spain, Australia, and the United Kingdom.
What does it mean, that we cannot show to our customers the delivery time, for fulfilment and delivery in Romania? I would expect more from Shopify to allow us to set manual transit time/delivery date, as the name of the option sais.
In conclusion I still need support or options to display at least an estimated delivery date to my customers, without this the online store doesn’t seem to be trustworthy and therefore our business has a lot to suffer.