At THIS moment it is possible in Sfakia to do a Covid-19 test every Tuesday. You need to make an appointment for this. If you can provide them with your health insurance ID number, it is free.
You will need a so-called Greek AMKA number, to get a free test.
To get a temporary AMKA number, as a foreigner: please read this:https://news.gtp.gr/2021/03/05/how-to-g ... ccination/
The article is about vaccination as a foreigner, but for tests only it is -at the moment- also necessary to get a AMKA number.
I am sure this procedure will be improved as soon as possible, when the tourist season opens. If I have news, I will post it here. Anyway your hotel host will know, and can inform you what to do, while you are there. It is in everybody's interest.
All the best,