If you'd like to change the error message that's being displayed to users when they encounter any issues during checkout:
You can do so by updating the Error Screen template, for example, as shown below:
HTML
<p class="ng-hide" ng-show="terminalError" ng-if="terminalError.indexOf('old error') > -1">
New message </p>
<p class="ng-hide" ng-show="terminalError" ng-if="terminalError.indexOf('old error') == -1">
{{terminalError}}
</p>
You need to provide the current error message or part of it as the argument ('old error') in the code above and replace it with a new copy or message.