Tenses - We use Present Perfect in this sentence because

We have run out of milk. I can't make tea for you unless I go out to buy more milk.

It’s an action that started in the past and continues in the present.

It’s an action performed during a period that has not yet finished.

It’s a repeated action in an unspecified period between the past and now.

It’s an action which has a consequence in the present.

I have already read 20 books this year. Isn't that amazing?

It’s an action that was completed in the very recent past

It’s an action when the time is not important.

I have lived in this city for more than 10 years now.

I have only been to Singapore once in my life. 

I have just come in. Can't you let me rest for a moment before we go out again?

He has called me 5 times already. Some people just can't take a hint.

I have lost my keys. I cannot get into the house. What should I do?