I need Cash On Delivery payment method when the customer selects a Store Pick up Shipping method during the checkout process. You can do this using “payment_method_is_active“ event:

Create an events.xml file at:
/app/code/Vendor/Modulename/etc/. Add the following code in this file:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="payment_method_is_active">
        <observer name="codpaymentdisableregion" instance="VendorModulenameObserverPaymentactive" />
    </event>
</config>

And create Paymentactive.php file at
Vendor/ModuelName/Observer/path. Add the following code in this file:

<?php
namespace VendorModuelNameObserver;

use MagentoFrameworkEventObserverInterface;

class Paymentactive implements ObserverInterface 
{
    protected $_checkoutSession;

    public function __construct(MagentoCheckoutModelSession $checkoutSession) {
        $this->_checkoutSession = $checkoutSession;
    }
    public function execute(MagentoFrameworkEventObserver $observer) {
        $objectManager = MagentoFrameworkAppObjectManager::getInstance();

        $_session = $objectManager->get('MagentoCheckoutModelSession');
        $method = $observer->getEvent()->getMethodInstance();
        $result = $observer->getEvent()->getResult();
        
        
        
        $shippingMethod = $_session->getQuote()->getShippingAddress()->getShippingMethod();
        
        if($_session->getQuote()->getShippingAddress()->getShippingMethod()=='storepickup_storepickup' && $method->getCode()=="cashondelivery")
        {
            return $result->setData('is_available', true);
        }
        
        if(strpos($shippingMethod,'ups_') !== false && $method->getCode()=="paypal_express")
        {
            return $result->setData('is_available', true);
        }
        
        return $result->setData('is_available', false);
    }
}

You need to replace “storepickup_storepickup” with your shipping code and “cashondelivery” or “ paypal_express” with your payment method code.