Argument 1: cannot convert from 'char' to 'string'

'StringBuilder' does not contain a definition for 'AppendJoin' and no accessible extension method 'AppendJoin' accepting a first argument of type 'StringBuilder' could be found (are you missing a using directive or an assembly reference?)

'StringBuilder' does not contain a definition for 'AppendJoin' and no accessible extension method 'AppendJoin' accepting a first argument of type 'StringBuilder' could be found (are you missing a using directive or an assembly reference?)

Argument 1: cannot convert from 'char' to 'string'

Process completed with exit code 1.