Override these views:

\vendor\elgg\elgg\views\default\errors\default.php

\vendor\elgg\elgg\views\default\errors\404.php

I looked at those files and I couldn't edit it enough to put out a message with a link to the front page.
However, I found the Message Error located at this address: elgg/languages/en.php
Page not found and  
The page you are trying to view does not exist or you do not have permissions to view it. "
 I can't change it because there is something stopping the change.
If you know which pages are putting out those "sentences" and if they can be edited?
Appreciate it.
Thank You.

If you need a redirect when there's no page,  you can use your server configuration rules.

If you want to customize error pages inside Elgg, use the previous my reply.

I don’t want to look for Apache config now (which I hate), but I’m sure Google will help you.

Here is the redirect configuration example for Nginx:

location ~* /my-old-page { rewrite ^ https://mycoolsite.com/my-new-page; }

Bonus link: https://htaccess.madewithlove.com/

This is not relevant to the question you asked.

All I can answer you here is that you cannot include non-existent functions that are not in the extended class.

For example, this code:

function curriculum_init() {}

Use

function init() {} 

instead.

Look at My plugin skeleton plugin

I think you need to learn more about PHP and Elgg or hire someone.

The Curriculum_int() function is working for I am able to register the menu...The java script in not loading

I used to program in Elgg 7-8 years back and am mostly a java developer....

I have not followed what has changed in Php and elgg and am confused a little, and so i am trying to understand it by little trial and error while trying to tweak my old app to the new design... This is how I can register what all has changed by asking questions...And some changes might not register

To hire a developer, I need to know what to ask for and so don't mind my questions....for I am still trying to figure out whether the design that worked in the past is relevant.. So don't mind my childish questions

I figured out a few things about Bootstrap Function or loading external javascript, but don't see the error logs to understand where the mistake is...

HelloWorld Plugin is basic and I understand this...

Do you have a tutorial to load an external javascript file in Bootstrap function...

And how much would a developer charge to convert my old app to the new one...

In addition to the official documentation, you can use the search on this community.

Also read these related Q&As:

https://elgghacks.com/questions/view/3947/require-js

https://elgghacks.com/questions/view/4088/amd-loading-js-issue

https://elgghacks.com/questions/view/5676/how-to-include-three-js-with-orbitcontrols-js-and-objloader-js-into-elgg

https://elgghacks.com/questions/view/5349/javascript-library

Please code example.

No code, no help ¯\_(ツ)_/¯

I was trying the Market Plugin that was there for 4.3 in 5.1...

I think its your code...Are you going to release for version 5.1, Even CkEditor extended has similar issue..

https://github.com/RiverVanRain/market...

Are you going to release for version 5.1

Sure. Coming soon...

https://github.com/Elgg/Elgg/blob/5.x/mod/discussions/views/default/plugins/discussions/settings.php

or

https://learn.elgg.org/en/stable/guides/menus.html