There are a view posts on the location of the config file for a linux installation. Most of them are outdated and have answers that no longer are correct.
The only way to find the config file
find ~ -type d -name Nextcloud
Why the client has no button edit config manually to stop the unneeded search action is beyond me.
please be clear about linux installation. If you’re referring to this: Installation on Linux — Nextcloud latest Administration Manual latest documentation please tell us which installation method was used.
If you’re referring to the snap method, see here Nextcloud snap wiki and support and here How to configure Nextcloud snap
crystal ball searches just don’t work yet
fwiw Using the Synchronization Client — Nextcloud Client Manual 3.7.50 documentation
tries to crystal ball but failed.
A link to the file in the client is the best option
mind changing the request heading?
Probably because 99% of users never need to change anything directly in the config file, and the developers probably don’t want to encourage less experienced users to do so. And as we can see, experienced users like you are perfectly capable of finding the file without such a button 
Also, if there really is a need for a significant number of “normal” users to change any of the additional settings in the config file, it would be better to expose them in the UI and/or change the defaults.