If you are not able to send Follow up messages you need to check your browser settings and enable push notifications in your browser.
How to enable push notifications in Google Chrome
How notifications work
By default, Chrome alerts you whenever a website, app, or extension wants to send you notifications. You can change this in settings at any time.
If you’re browsing in Incognito mode, you won’t get notifications.
To allow or block notifications in your Chrome browser,
Go to your Chrome browser.
At the top right corner, click More > Settings, or if you’re a Mac user, from the menu bar at the top, select Chrome > Preferences
Go to Advanced > Privacy and Security
Click Site settings.
Choose to block or allow notifications:
To block all notifications: Turn off Ask before sending.
To block a specific website: Next to "Block," click Add. Enter the site and click Add.
To allow a site: Next to "Allow," click Add. Enter the site and click Add.
You can go to 'View Site Information' in your Address bar and choose to block or allow notifications
You can also block any sites or apps from sending you notifications.
How to enable push notifications in Firefox
If you’re a MAC user,
Go to the menu, select Firefox > Preferences
Go to Privacy and Security
Click Allow or Block next to the website.
In your Address bar go to Site Information > Show Connection Details > More Information > Permissions and under 'Send Notifications' you can allow or block notifications.
If you’re a Windows user,
Click on the hamburger menu icon in the top right corner next to the home icon
Select Options > Content
Click the ‘Choose’ button next to Notifications
Here you can see a list of websites for which you’ve allowed notifications. You can remove the sites from which you don’t want to get push notifications.
How to enable push notifications in Safari
Go to Safari in the top menu
Go to Websites > Notifications
You will find the websites listed where you can select Allow or Deny to enable or disable notifications from the website.
How to enable push notifications in Microsoft Edge
Click on the More icon in the top right corner
Go to Settings > Advanced
Click on the website for which you want to enable/disable push notifications
Move the toggle to the right to enable and to the left to disable notifications