I am trying to create a Record Type from scratch but it opens a page and says that "The data source cannot be reached or its configuration is invalid. Please contact your system administrator.". what should be done ?

As in Community Edition we don't have much control over the data source setup, suggest you to send and inform to communityedition@appian.com so that Appian team can take a look. 

Community Edition generally has this issue and support team revert you soon with exact resolution.
Write to communityedition@appian.com along with screenshot.