Hi Przemek Kasierski
Which Cloud Service you are trying to deploy
Cloud Services (extended support) doesn't support the logical concept of a hosted service, which included two slots (Production & Staging). Each deployment is an independent Cloud Service (extended support) deployment. To test and stage a new release of a cloud service, deploy a cloud service (extended support) and tag it as VIP swappable with another cloud service (extended support).
Refer to this frequently asked questions documents - https://docs.azure.cn/en-us/cloud-services-extended-support/faq
Hope this helps!
Please Let me know if you have any queries.
If you found the information helpful, please click "Upvote" on the post to let us know and consider accepting the answer as the token of appreciation. Thank You.