Magento 2 Email – Order Confirmation Emails Not Sending

Once I place an order, confirmation email is not sent.

In the backend (Sales -> Orders), the order says that "The order confirmation email is not sent".

If I click "Send Email", it still doesn't send.

Other mail functionality works (e.g. Forgot Password). Emails are configured under Stores -> Configuration -> Sales -> Sales Emails. Under Advanced -> System, my Mail Sending Settings are as so:

Cron job is setup and running four times every hour, as so:

system.log: https://pastebin.com/X2Vm0trx

update.cron.log:

Could not open input file: /home/[user]/public_html/update/cron.php
Could not open input file: /home/[user]/public_html/update/cron.php
Could not open input file: /home/[user]/public_html/update/cron.php
Could not open input file: /home/[user]/public_html/update/cron.php
Could not open input file: /home/[user]/public_html/update/cron.php
...

update.log:

[2018-09-02 22:14:02] setup-cron.ERROR: Could not locate magento/magento2-base/composer.json file. [] []

I suspect it's from a conflict with one of my installed extensions, given the error in my system.log:

[2018-09-02 23:52:02] main.ERROR: Notice: Undefined property: MageArray\OrderAttachments\Model\Order\Email\SenderBuilder::$messageManager in /home/[user]/public_html/app/code/MageArray/OrderAttachments/Model/Order/Email/SenderBuilder.php on line 75 [] []

But there are also those other errors that are present, and I don't know if those errors are the ones causing the order confirmation emails to not be sent.