Your JSON file contains three lines, none of which are valid JSON. Each of the first two lines is almost valid JSON. There are two approaches you could take, one is to fix up the lines you have using mutate before you try to parse it
mutate {
gsub => [
"message","\]","",
"message","\[","",
"message", "^,", ""
]
}
if [message] =~ "^$" { drop {} }
json {
source => "message"
target => "event"
}
The other is to parse the entire file as an array of JSON objects, then use a split filter to separate them. See here for more information.