NameError: name 'target_id' is not defined

[DeaD_EyE]

The problem comes from create_target.
If the condition is not fulfilled, the target_id is not assigned.
You should update Python to 3.6 or later.
Python 3.5 and 2.7 are End of Life: https://devguide.python.org/#status-of-python-branches

If you put your code into function, it's later easier to test.
Here your function:

def create_target(ip_address):
    xml_data = (
        f"--xml=<create_target><name>Subnet {ip_address} "
        f"</name><hosts> {ip_address} </hosts></create_target>"
    )
    cmd = ["omp", "--username", "admin", "--password", "admin", xml_data]
    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=False, encoding="utf8")
    stdout, stderr = proc.communicate()
    if "OK" in stdout:
        print("target was created, target id:")
        target_id_tuple = re.findall(r'\w+(?:-\w+)+', stdout)
        target_id = target_id_tuple[0]
        print(target_id)
        return True
    elif "Failed to acquire socket." in stderr:
        print("omp service is not running")
    else:
        print(stdout, stderr)
        return False