If you have changed the country setting (as you say you have) then it needs to be set to Greece in order to get access to the Greek store. The article Change your Apple ID country or region - Apple Support describes how to change your Apple ID country or region, and the requirements and results of doing this.
Here are various issues and how to deal with them:
● Subscriptions
Make sure you have canceled all your subscriptions (see: If you want to cancel a subscription from Apple - Apple Support ). Wait for any memberships, pre-orders, movie rentals, or season passes to complete. Items will still be active until their expiration date. You need to contact Apple and request any ongoing items you have be ended immediately if you do not want to wait until they expire naturally.
● Remove all your Apple Account balance
- In the main change country article Apple only proposes a single way for you to clear your balance. That is to contact them and ask them to keep it. Apple offers this specific link for that service --> https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0998&symptom_id=20198
- Another still viable way is to add a credit or debt card as a payment method to your Apple ID and buy items just in excess of your remaining Apple ID balance. This will use up all the balance and any excess will get charged to the card, leaving nothing in your account.
● Pending purchases and refunds
If you have pending purchases, or refunds in the process of being returned to you, you will have to wait until these clear.
● Unusual case where having a child on Family Sharing may have prevented changing countries --> https://discussions.apple.com/thread/253296611
The recommended solution was to contact Apple.
For items where you need to contact Apple to resolve an issue:
Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
See a list of Apple phone numbers around the world. Click here --> Contact Apple for support and service - Apple Support