Is there's a way to get access to any Test environment or download a VM for

Hi Saket,
You can install the Appian in your own system following the link as shared below :
forum.appian.com/.../Installation_Guide.html
Apart from that another way to practice is to get registered to the Appian Virtual Coached academy. You can get there from Actions -> Appian Training - > Register for online training.
Then go through the Lead Designer course(300,301,302) this will be useful for Appian Lead Designer certification.

I believe that you need to get in touch with either your Account Executive (if you're an Appian Customer) or Partner Representative (if you're a Partner).