The error message “Type ‘undefined’ cannot be used as an index type” in TypeScript is thrown when the compiler is unable to infer the type of an index signature. An index signature is a way of specifying the type of a property in an object, and it is used when the property is accessed using the square bracket notation, like object[property].

For example, if you have an object myObject and you try to access a property that does not exist, you will get this error message.

const myObject = {
name: "John",
age: 30
};
console.log(myObject["gender"]);

In the above example myObject doesn’t have property of “gender” so while accessing it will throw error “Type ‘undefined’ cannot be used as an index type”

To fix this error, you have a few options:

• Make sure the property you are trying to access exists in the object
• Use the type any for the object, which will tell the compiler to ignore the error
• Use the --noImplicitAny flag when compiling your TypeScript code, which will force you to specify the types of your variables and properties
• Use the TypeScript's indexable types which allows to define the type of the properties of an object, by providing a key and value type. For example, an object with string keys and number values can be defined as { [key: string]: number; }.

It’s important to understand that the error message itself means that the compiler cannot infer the type of the value that is being accessed by the property. This can happen when the property is not defined or is undefined, and the compiler doesn’t know how to handle it.