That error message is correct. There is no “replaceAt” method in JS. You would have to define it yourself or import it from a library.
I see this is solution 1 in the hints. The explanation makes reference to " modifying the replaceAt function using prototype" but this code isn’t included in the solution. So unless it is provided by default during testing (which it doesn’t appear to be) then it won’t work, which is why you are getting this error.
Also, I don’t think these are considered “official” solutions. I believe they are just possible solutions contributed by members over the years. Perhaps something was different back when this solution was contributed and a replaceAt method was provided? Or perhaps there originally was additional code that defined a replaceAt method and it has been removed over time?