I suspect you’re using an old version of Mojo. The out keyword was added in 24.6, which was released last week.
On 24.6, I get the same error as you if I use a non-existent keyword in place of out:
struct MyPair:
var first: Int
var second: Int
fn __init__(potato self, first: Int, second: Int):
self.first = first
self.second = second
error: expected ')' in argument list
fn __init__(potato self, first: Int, second: Int):
^
Notice that the terminal puts an arrow below the token that it doesn’t expect to encounter. I expect for you, the arrow is also pointing to self. If so, this is because out is not a keyword in the version of Mojo that you’re using.
This error message isn’t very informative, so I’d filed a Github issue to improve it.