Your JSON does not follow the correct syntax for messages and can therefore not be displayed by Slack.
You need to include your JSON for the blocks under a property blocks in your message definition - on the same level like channel. Your JSON for blocks also needs to be an array.
Working example:
{
"channel": "XXXXXXXXXX",
"blocks":
[
{
"type":"section",
"text":{
"type":"mrkdwn",
"text":"You can add a button alongside text in your message."
},
"accessory":{
"type":"button",
"text":{
"type":"plain_text",
"text":"Button",
"emoji":true
},
"value":"click_me_123"
}
}
]
}
See this link for the reference documentation on "Putting blocks in messages"