Yeah, there are some limiations live connected to ssrs, we can't create measure and calculated column. You'd better use import model. Or you create a viusal, add all the needed data into the table. Then export the data into csv file, open another .pbix file and upload the data from csv file, and create measure and calculated column there.
I want to find out if the Accounts has a sales person associated with it then it should show Available But if the Account name has unknown or blank associated with it then show unavailable.
It's hard to post detailed solution without sample table. If there is relationship between the table? You'd better share the sample table and list expected result for further analysis.