Got a warning message:
...when opening up Sas Studio. I do not pay for it (it has been free to use, have been using it for half a year at least) Not sure what should I do, because I would like to keep using it and I would like to keep all my documents. Is the SAS Studio just a trial version? What do I need to do? Can I not use it after April 4? or should I just ignore it?

Ignore it. Just logged in to SAS On Demand and saw the same message. SAS will update the license and the message will go away.
https://communities.sas.com/t5/SAS-Software-for-Learning/Please-ignore-expiration-warning-for-SAS-OnDemand-for-Academics/m-p/858792