Method prefix does not exist (?)

@mesqueeb This is the same error you were stuck on right?

You see whats weird about this is that fluent routing wasn't introduced until 5.4 so i'm pretty sure you couldn't define a prefix() method in previous versions.

If i'm correct then for some reason, 5.4 features are being pulled in.

You could change the RouteServiceProvider and remove the prefix method and just define the prefix as you usually would on a route group and see if that works.

Route::group(['prefix' => 'api'], function () {
    // require routes.
});

Thanks for responding. I'm not terribly fluent in routing, and don't this business with route groups. I used to have one reasonably straightforward routes file that took care of my needs. Part of me almost wants to continue the upgrade to 5.4. But I thought I'd stop for a bit at 5.3 and make sure everything is working before moving on. I'll try your suggestion out if I can get the syntax right.

Grrr... the more I try to fix it, the leaks get sprung. The errors keep changing, but there's always a new one. Feels like I'm going down a rabbit hole. The latest one is [Symfony\Component\Debug\Exception\FatalErrorException] Trait 'Illuminate\Foundation\Auth\AuthenticatesAndRegistersUsers' not found

Here's my edited mapWebRoutes method:

  protected function mapWebRoutes()

{ Route::group(['middleware' => 'web', 'namespace' => $this->namespace], function () { require base_path('routes/web.php'); }); }

I'm sure that this isn't right. but I'm starting to run out of patience with this.

As i know there is a ($router) within the function in RouteServiceProvider in Laravel 5.3. Have you try,

Route::group([
  'middleware' => 'web',
  'namespace' => $this->namespace,
  ], function ($router) {
       require base_path('routes/web.php');
});

Just to follow up, using @hendranucleo's suggestion got me rolling (thank you!). It was still no picnic after that, as there are a number of changes not mentioned in the upgrade instructions that I had to account for. It took most of a day to go through each new error and figure out whether a key file had either changed and had to be replaced with a fresh one, or had to be added, etc.

Can you post your mapApiRoutes method in RouteServiceProvider ?

Route::group([
  'middleware' => 'api',
  'namespace' => $this->namespace,
  'prefix' => 'api',
], function ($router) {
  require base_path('routes/api.php');
});

Do you have this line after namespace on RouteServiceProvider ?

<?php
namespace App\Providers;

use Illuminate\Support\Facades\Route;
use Illuminate\Foundation\Support\Providers\RouteServiceProvider as ServiceProvider;

yes.

How you upgrading your laravel? copy paste folder or using composer?

I used Composer, but, as I wrote previously, I still had to make a number of manual changes. New routes directory, new auth controllers, etc., etc. Everything seems to be fine now, but it took more work than I expected.

Something to do with composer.json. I had this problem also upgrading from 5.3 to 5.4.

Try renaming current composer.json to something else; copy in a vanilla one from 5.3; run composer update; then copy in your own additions to the require and require-dev section. That fixed it for me.