Property or method "data1" is not defined on the instance but referenced during render.

This error typically occurs when you are trying to access a property or method that is not defined in the Vue component's data object or methods section.

To resolve this issue, you need to make sure that the "data1" property is defined in the Vue component's data object. For example:

data() {
  return {
    data1: 'some value'
  }
},

Alternatively, if "data1" is a method, you need to define it in the methods section of the Vue component. For example:

methods: {
  data1() {
    // implementation
  }
},

Make sure that you are referencing the property or method correctly in your component's template. If you are using the property or method in a template expression, make sure to use the correct syntax (e.g., `{{ data1 }}` for property access and `{{ data1() }}` for method invocation).