There are four ways to install and activate the Mailchimp Block on your WordPress site.
1st Way: Install By Search
Login to your WordPress Admin area
Click on the Plugin menu on the left side
Now, go to the right side and search for the “Mailchimp block” plugin
Click on the Install Now button
Once the plugin is installed, click on the Activate button

Now, you can show in your Admin Panel, the new menu has been added Mailchimp Block.
2nd Way: Install from Gutenberg Editor
Go to your Gutenberg Editor.
Click on the Plus button from the top left corner.

Search by Mailchimp
Click on Mailchimp Block Plugin

3rd Way: Install By Upload Plugin
Download the latest version of the Mailchimp-block Plugin (.zip file)
Login into your WordPress Admin area
Click on the left-side Plugin menu
Now at the top of the page, you can see the Add New button, click on that buttons
Again at the top of the page, you can see the Upload Plugin button, click on that button for the next process
Click on “Choose File“
Now upload the Plugin zip file MailChimp-block and click on the “Install Now” button
Activate the Plugin

4th Way: Install Using FTP
Extract the above-selected zip file (MailChimp-block.zip). It will create a “MailChimp-block” folder with all Plugin files and folders inside it
Using your FTP programs like Filezilla or cPanel, upload the non-zipped Plugin folder into the /wp-content/plugins/ folder on your server
Go to the Dashboard » Plugins » Installed Plugins page and find “Mailchimp-block” from the Plugin list
Click on Activate Button to activate the Mailchimp Block Plugin
Now, you can show in your Admin Panel, the new menu is added Mailchimp-block