getting error when doing the composer install
I think the composer error messages are really opaque: Took me the whole day to figure this one out. And stil am not sure how to debug the composer related problems (and I have read all the drupal documentation, lots of composer documentation, so maybe I am just losing IQ points lately...)

Error 1: composer install

I had the same problem on composer install (Clive removed my earlier remark) and thought the culprit was 

remove drupal/drupal No version set (parsed as 1.0.0)

Couldn't figure out why that would happen: Added all modules neatly with require bladiebla and had no problem updating core to 8.6.4

Site was running smoothly

Error 2: install module

Trying to install backup_migrate resulted in the error

The package is not available in a stable-enough version according to your minimum-stability setting

Which I also did not understand because my minimum stability was dev and the backup_migrate version was a stable one

Trying different things to find out where the problem originates did not bring any solution

composer install
composer update
composer diagnose
composer why
composer give me a break
etc

Is supposed that one of the modules I required conflicted with another one, but how to find out which one..... (any hints on how to debug these composer message are welcome)

SOLUTION (in my case)

In the end I simply did this

starting point

drupal 8.6.4

minimum stability dev

php 7.0.3

actions


remove core, modules/contrib, vendor, composer.lock
in composer.json: 


2.1 start with (in my case)

    "drupal/core": "~8.6.0",
    "composer/installers": "^1.0.24",
    "wikimedia/composer-merge-plugin": "^1.4",


2.2 in the following require part require all advised highest version for all modules you use (so go to the drupal mordules site en copy the advised composer require statements)

   "drush/drush": "~9",
    "drupal/backup_migrate": " ^4.0",
    "drupal/backup_db": " ^1.1",
    "drupal/draggableviews": " ^1.0",
    "drupal/pathauto": " ^1.0",
    "drupal/token": " ^1.0",
    "drupal/anonymous_redirect": "^2.2",
    "drupal/ctools": " ^3.0",
    "drupal/calendar": " ^1.0",
    "drupal/geolocation": "^2.0",
    "drupal/views_taxonomy_term_name_depth": "6.x-dev",
    "drupal/views_templates": "^1.0",
    "drupal/paragraphs": " ^1.5",
    "drupal/devel": "^1.2",
    "drupal/webform": " ^5.0",
    "drupal/social_login": " ^2.83",
    "drupal/disqus": " ^1.0",
    "drupal/address": " ^1.4",
    "drupal/leaflet": " ^1.0",
    "drupal/geofield": "^1.0",
    "drupal/geophp": "^1.0",
    "drupal/geocoder": "^2.0",
    "drupal/geofield_map": "^2.20",
    "drupal/google_analytics": "^3.0",
    "drupal/video_embed_field": "^2.0",
    "drupal/better_exposed_filters": "^3.0",
    "drupal/addtoany": "^1.10",
    "drupal/typed_data": "^1.0",
    "drupal/rules": "^3.0",
    "drupal/honeypot": "^1.29",
    "drupal/field_group": " ^3.0"



run 
composer install
go to your site and run www.domain.ext/update.php


That resolved my problem