When we locally checked for the issue, we were unable to find such kind of issue i.e the plugin is working fine.
Further, we actually do not know the cause of error in your site so it might be due to conflict with another third party plugin that you may have activated. So please once try to check the issue by only activating WP Travel and WP Travel Express checkout plugin.
If the issue still persists then we need to check your specific setup and find the cause of error.
So, in that case, you need to provide your login detail in the email given below: