Why am I getting this 404 error?

Here's my method:

public function update(Request $request)
    {
      $test_results= CeuResult::findOrFail($request->the_quiz_id);
      if($request->pass <> $test_results->pass) {
         if($request->pass==1) {
            $test_results->attempts=1;
            $test_results->quiz_date=date('Y-m-d');
            $test_results->status='PASSED';
            $test_results->pass=1;
            $test_results->save();
         }
         else {
            $test_results->attempts=0;
            $test_results->pass=$request->pass;
            $test_results->quiz_date=NULL;
            $test_results->status=NULL;
            $test_results->save();
         }
      }
      
      $customer=CeuUser::findOrFail($request->the_id);
         //update ceu_users table
         $customer->winclose=$request->winclose;
         $success='User was successfully updated!';
         
         if($customer->save()) {
            Session::flash('message', 'User was successfully updated!');
            return redirect('/admin/show/cuser/'.$request->the_id);
         }   
    }

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Here's a screenshot of the site we're looking at:

What I need to do is update the "Window Close" date. 

That will happen at this point in the code:

$customer=CeuUser::findOrFail($request->the_id);
         //update ceu_users table
         $customer->winclose=$request->winclose;
         $success='User was successfully updated!';

The thing is, when I click on submit, I get a 404 error. 

My routes look like this:

    Route::get('/list/cuser', 'CUserController@index')->name('adminListCUsers');

   Route::get('/show/cuser/{id}', 'CUserController@show')->name('adminShowCUser');

   Route::post('/update/cuser', 'CUserController@update')->name('adminUpdateCUser');

   Route::post('/update/cuser/status', 'CUserController@update_status')->name('adminUpdateCUserStatus');

   Route::post('/delete/cuser', 'CUserController@destroy')->name('adminDeleteCUser');

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I can do this:

public function update(Request $request)
    {

     echo "hello";

...and get a "hello," so this:

Route::post('/update/cuser', 'CUserController@update')->name('adminUpdateCUser');

...is working fine. 

But something's going south within the function housed in the Controller and I'm not even sure how to troubleshoot it. 

Why would I get a 404? How do I figure out what's wrong?