How to fix function which I'm trying to pass into "payload" in bash ? I've stuck with error "invalid payload"

I'm trying to configure custom slack notification from GitLab but keep getting error in GitlabCI. I'm wondering what I'm doing wrong? Maybe that's an error in my function? Btw I've tested the weebhook itself and it's valid and working as expected

This is a piece from my .gitlab-ci-.yaml

  ENVIRONMENT_NAME: "staging"
  extends: .Deploy_image
  only:
    - master
  after_script:
    - if [ ${CI_JOB_STATUS} == "success" ]; then EXIT_STATUS=0; else EXIT_STATUS=1; fi
    - envsubst </tmp/${CI_PIPELINE_ID}/${SCRIPT_PATH}/slackscript.sh> "slackscript.sh"
    - source ./slackscript.sh; share_slack_update_deploy

and the script itself:

#!/bin/bash

set -euo pipefail

FAILURE=1
SUCCESS=0
SLACKWEBHOOKURL="my webhook url"

function print_slack_summary_deploy() {

    local slack_msg_header
    local slack_msg_body
    local slack_channel
# Populate header and define slack channels
slack_msg_header=":x: *Deploy to ${ENVIRONMENT_NAME} failed*"
if [[ "${EXIT_STATUS}" == "${SUCCESS}" ]]; then
        slack_msg_header=":heavy_check_mark: *Deploy to ${ENVIRONMENT_NAME} succeeded*"
        slack_channel="channel-name"
    fi
cat <<-SLACK
            {
                "blocks": [
                    {
                        "type": "section",
                        "text": {
                            "type": "mrkdwn",
                            "text": "${slack_msg_header}"
                        }
                    },
                    {
                        "type": "divider"
                    },
                    {
                        "type": "section",
                        "fields": [
                            {
                                "type": "mrkdwn",
                                "text": "*Stage:*\nDeploy"
                            },
                            {
                                "type": "mrkdwn",
                                "text": "*Pushed By:*\n${GITLAB_USER_NAME}"
                            },
                            {
                                "type": "mrkdwn",
                                "text": "*Job URL:*\nGITLAB_REPO_URL/${CI_JOB_ID}"
                            },
                            {
                                "type": "mrkdwn",
                                "text": "*Commit Message:*\nGITLAB_REPO_URL/${CI_COMMIT_MESSAGE}"
                            {
                                "type": "mrkdwn",
                                "text": "*Commit URL:*\nGITLAB_REPO_URL$(git rev-parse HEAD)"
                            },
                            {
                                "type": "mrkdwn",
                                "text": "*Commit Branch:*\n${CI_COMMIT_REF_NAME}"
                            }
                        ]
                    },
                    {
                        "type": "divider"
                    }
                ]
}
SLACK
}

function share_slack_update_deploy() {

local slack_webhook

slack_webhook="$SLACKWEBHOOKURL"

curl -X POST                                           \
        --data-urlencode "payload=$(print_slack_summary_deploy)"  \
        "${slack_webhook}"
}