Excel class 'app excel' not found

Describes the cause and action for error messages.

Error Message(English)：
class 'app excel' not found

Resolution

Class 'Maatwebsite\\Excel\\ExcelServiceProvider' not found ...
https://stackoverflow.com/questions/54083863/class-maatwebsite-excel-excelserviceprovider-not-found-・・・

Class 'Maatwebsite\Excel\Excel' not found
https://stackoverflow.com/questions/50551149/class-maatwebsite-excel-excel-not-found

"Class 'App\Http\Controllers\InvoicesExport' not found" in ...
https://stackoverflow.com/questions/50246497/class-app-http-controllers-invoicesexport-not-found-in-・・・

Class 'CI_Excel' not found
https://stackoverflow.com/questions/41734173/class-ci-excel-not-found

Laravel 5.2 で Excel出力 - Qiita
https://qiita.com/UedaTakeyuki/items/0255ed7c9ccdcd83263b

Laravel Excelをインストールする方法 ? かきくけ子のブログ
http://kakikukeko.sakura.ne.jp/blog/how-to-install-laravel-excel/

Followed instructions but still Class 'Excel' not found ・ Issue #510 ...
https://github.com/Maatwebsite/Laravel-Excel/issues/510

[BUG] Class excel does not exist ・ Issue #1152 ・ Maatwebsite/Laravel ...
https://github.com/Maatwebsite/Laravel-Excel/issues/1152

CakePHP - cakephp3でPHPExcelを使ってexcelのリストをデータベースに ...
https://teratail.com/questions/60234