Facing exactly the same issue with MAP 9.9.13 on SQL 2016 express on Win 2012 R2. First created the instance "MAPS". Install failed with error : "Assesrtion Failed", Cant repair or clean unsintall.
Tried uninstalling via Uninstall String in Registry however that says product not found. MAP toolkit appears in the installed software though but then while launching gives error that "A failure occured while validating the MAP Toolkit configuration
database.
Uninstalled the localdb "MAPToolkit" and deleted the residue folder "C:\Users\"current user"\AppData\Local\Microsoft\Microsoft SQL Server Local DB\Instances\MAPToolkit"
Tried Reinstalling MAP TK but same "Assertion Failed" Error. Checking internally.