<group id="braintree_required" translate="label" showInDefault="1" showInWebsite="1" sortOrder="5">
<comment><![CDATA[<a href="https://www.braintreegateway.com/login" target="_blank">Click here to login to your existing Braintree account</a>. Or to setup a new account and accept payments on your website, <a href="https://apply.braintreegateway.com/signup/us" target="_blank">click here to signup for a Braintree account</a>.]]></comment>
<label>Basic Braintree Settings</label>
<attribute type="expanded">1</attribute>
<frontend_model>Magento\Config\Block\System\Config\Form\Fieldset</frontend_model>
<field id="title" translate="label" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
<label>Title</label>
<config_path>payment/braintree/title</config_path>
</field>
<field id="environment" translate="label" type="select" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="0">
<label>Environment</label>
<source_model>Magento\Braintree\Model\Source\Environment</source_model>
<config_path>payment/braintree/environment</config_path>
</field>
<field id="payment_action" translate="label" type="select" sortOrder="50" showInDefault="1" showInWebsite="1" showInStore="0">
<label>Payment Action</label>
<source_model>Magento\Braintree\Model\Source\PaymentAction</source_model>
<config_path>payment/braintree/payment_action</config_path>
</field>
<field id="merchant_account_id" translate="label" sortOrder="80" showInDefault="1" showInWebsite="1" showInStore="0">
<label>Merchant Account ID</label>
<config_path>payment/braintree/merchant_account_id</config_path>
</field>
<field id="merchant_id" translate="label" sortOrder="90" showInDefault="1" showInWebsite="1" showInStore="0">
<label>Merchant ID</label>
<config_path>payment/braintree/merchant_id</config_path>
</field>
<field id="public_key" translate="label" type="obscure" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="0">
<label>Public Key</label>
<config_path>payment/braintree/public_key</config_path>
<backend_model>Magento\Config\Model\Config\Backend\Encrypted</backend_model>
</field>
<field id="private_key" translate="label" type="obscure" sortOrder="110" showInDefault="1" showInWebsite="1" showInStore="0">
<label>Private Key</label>
<config_path>payment/braintree/private_key</config_path>
<backend_model>Magento\Config\Model\Config\Backend\Encrypted</backend_model>
</field>
</group>
So you should fill all the fields of the group «Basic Braintree Settings » to enable the «Enable this Solution » control.
On the screenshot above the field «Merchant Account ID » is empty and should be filled.