It can be very time-consuming and frustrating to write the MDX necessary to connect to SAP-BW queries. There is a transaction in SAP (MDXTEST) which might be helpful, but often the SAP Basis administrators do not want or are unable to grant access to this transaction.
One of the best tools I have found is the "Microsoft Report Builder" which is part of the SQL Server/BI tools but freely downloadable on its own. This gives you a point-and-click interface to your BW queries and will convert them to nice, easily readable MDX which you can just copy and paste into Qlik Sense.
The only thing left to do then is lots of statements like;
"[Measures].[CBGV9LKY9N4J3F6DFJUQ83NTY]" as "Invoice Value",
to convert them into readable column headers.