Not sure if I can help you here. It seems there's a plugin at https://github.com/hypeJunction/Elgg-acl_builder_api/releases that would provide the hypeJunction\Access\Collection class. But the available releases of this plugin are for Elgg 1.11/1.12, so it might no longer be required on Elgg 2.x (maybe the functionality is provided by another plugin by now).

Are you getting the error on every message you are trying to send or only when friending? Is the friending functionality connected with a message sending option on your site (by default it isn't so I wonder if you use some 3rd party plugin that adds some functionality to send a message together with a friend request)? If you are using some other plugin for friending, there might be a conflict with this plugin in connection with the hypeAttachments plugin. I don't use this plugin myself - actually I have problems understanding how the hypeAttachments plugin is even supposed to work exactly (maybe some additional plugin are required I have not yet discovered).

If you can reproduce the error and it's connected with hypeAttachments (and you don't want to stop using this plugin), you might want to report an issue at https://github.com/hypeJunction/hypeAttachments/issues and you maybe get some support by the developer of the hypeAttachments plugin or at least some hint what you might need to do to prevent the error from occuring.

acl_builder_api is no longer a plugin but a composer dependency and should be in the dist package of hypeAttachments or hypeInbox. If not, please report on github

if you use an input/longtext in your code and you have a WYSIWYG plugin enabled you'll get an editor.

Elgg comes bundled with CKEditor, but here on the community you can also find a TinyMCE plugin.

To display the contents the user entered correctly you need to use an output/longtext.

Have a look here to learn about views http://learn.elgg.org/en/stable/guides/views.html