failed to combine variables, expected dicts but got a 'dict' and a 'AnsibleUnicode'

ERROR! failed to combine variables, expected dicts but got a 'dict' and a 'AnsibleUnicode':
{}
"ansible_user=root ansible_pass=rootroot"


group_vars - change to below code

ansible_connection=ssh
ansible_user=vagrant
ansible_ssh_pass=vagrant

ref: https://serverfault.com/questions/628989/how-to-set-default-ansible-username-password-for-ssh-connection