Your site doesn’t include support for the “*” block. You can leave it as-is or remove it

You’re getting that notice because your JavaScript code resulted in the following error:

ReferenceError: Cannot access ‘EditComponent’ before initialization

This means you need to register your block after the constants EditComponent and SaveComponent are defined.

// Define the constants first.
const EditComponent = ...;
const SaveComponent = ...;

// Then register your block.
registerBlockType('eternaltwentyfifteen/prevnext', {
    // Your args here.
    edit: EditComponent,
    save: SaveComponent,
});

Alternatively, you can declare them as functions instead.

// Register the block.
registerBlockType('eternaltwentyfifteen/prevnext', {
    // Your args here.
    edit: EditComponent,
    save: SaveComponent,
});

// Functions are hoisted, so EditComponent and SaveComponent are already available
// before registerBlockType() is called.

function EditComponent() {
    ...
}

function SaveComponent() {
    ...
}

Further Reading

Excerpts from MDN:

• const declarations can only be accessed after the place of declaration is reached (see temporal dead zone). For this reason, const declarations are commonly regarded as non-hoisted.

  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const#description

• Functions must be in scope when they are called, but the function declaration can be hoisted (appear below the call in the code).

  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions#calling_functions

See also Understanding JavaScript Function Declarations: ‘function’ vs ‘const’ on DEV.to.