You can travel on 4 days.

On 2 of those 4 days, you can travel in your home country.

I can or I must? Because I will go by plan to spain and come back with the global pass. Is there any cheaper possibilite for me now because I only need 2 days of travelling and today I can still cancel my global pass for 4 days.

Am I right...?

And how do I cancel...?

There's no obligation to use your inbound/outbound journeys. You can also use your 4 days elsewhere.

If you only need 2 days, then it may be cheaper to buy normal tickets. You'll have to look up prices for those and compare with your pass + reservations.

Refunds are explained here:

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

My global pass starts at 27.03. Can I postbone the datefor later...? I will use it  1 months  later !

Yes, you can deactivate the pass today until 23.59 CEST. You must activate your pass at the latest 11 months after purchase.

Only if it's a mobile pass… if it's a paper pass, you can only request a refund since the start date is fixed.

Do you have a paper pass or mobile pass?

Mobile pass

OK, I hope you cancelled the activation on the 26th. Please never activate a mobile pass in advance. There is no advantage in doing so.