Hi @oliviawd
I understand the problem you are facing !
As known as i know - It doesn't matter how you have installed magento using Git or using composer !
But in your magento 2 setup you required to check - composer is installed or not ?
You can check by running this command - composer -v
using above command you will able to see composer version - if its installed or not !
Also i have seen in your command you haven't passed any magento version - so there might be a chance due to that you are not able to upgrade your magento .
Kindly follow the below command :
composer require magento/product-community-edition 2.1.15 --no-update
Then you required to run below command in sequence :
rm -rf var/di/* var/generation/* var/cache/* var/log/* var/page_cache/* var/session/* var/view_preprocessed/* pub/static/*
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy
php bin/magento cache:clean
php bin/magento cache:flush
php bin/magento indexer:reindex
Hope it helps !
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