Couple problems there. One of them is a little sneaky and not the source
of your error  but you should be aware of it. The declaration of your
variables will result in a and b being Typed as Variant and c as a string.

The source of the reported problem, though, is that you've no spaces
before or after the ampersand.

Sub MakeAString()

Dim a As String
Dim b As String
Dim c As String

a = "abc"
b = "def"
c = a & b & "another string"

Debug.Print c

End Sub