Please consider thoroughly if you want to use Multisite. It has never been officially supported, probably never fully worked and it is definitely no longer supported on Elgg 3 at all! If you use Multisite, you are stuck with Elgg 2 and with the release of Elgg 4 (probably not happen too soon but surely one day) you will no longer get any support for Elgg 2 (not even security fixes) and are at a dead end with Multisite.

I can't help with getting Elggchat to work with Multisite (I see it as time wasted for nothing to be honest). I just wanted to make this clear. I hope you'll find someone to do the job if you really want to stick with Multisite. But maybe it would be better to try it without it.

Dear Linda,

i totally agree with @iionly. Multisite (if that really is what you use) is no longer part of Elgg (since v3). You seriously need to consider updating your site. That way, if there are issues, you have more change to get help/support on this community.

I totally agree with @iionly and @jeroen

While I am also not a support of multisite installation ( which is not supported by elgg) I have done a small setup for my multisite project by using VestaCP API for database and vhost generation and use .htacess to redirect request to the central elgg instance , that way I can kinda use multisite without effecting elgg itself 

Note: each of my multisite vhost have there own elgg config file automatically generated via PHP script , hope this helps