First, connect with the Shopify Customer care
Also contact the party (GoDaddy or Namecheap) from where you bought the domain, because if it is already owned by others you can’t avail it. So try to connect with them
It sounds like your domain had been purchased by another person before and he/she used it for another Shopify store. And after the domain expired, he/she didn’t remove it from the store.
You can connect to Shopify Support and hopefully, they will be able to remove it from the previous store somehow.
Issue is fixed by shopify support. It seems domain is already connected but somehow there wasn’t any information about it on shopify side. Everything is okay now. Thanks for your help.