Hey there @VALERIE50677 - the requirements for our Outlook Plug-in are: 
Operating system - Windows only. Outlook 2007, 2010, 2013 or 2016 or 2019, with Windows XP, Vista, 7, 8, 8.1 or 10 or 11.

Outlook 2021 will not be compatible. That said - you should absolutely check to see if you can connect via our O365/Outlook.com calendar connection! If you can view your calendar here, we would advise connecting using our Office 365/Outlook.com connection. This will lead to a more seamless experience with Calendly and should only take a few minutes to set up. If this is not an option, we recommend using the plug-in. We also recommend the plug-in if you have specific security requirements that prevent you from using a cloud-based calendar application.

Check out this Help Center article on our Outlook Plugin that goes over the requirements as well as alternatives. Here is some info from that article: 

The Outlook Plug-In, sometimes called the Calendly Plug-in for Outlook or the Outlook Desktop Plug-In, is a Windows application that connects your local Outlook desktop calendar to your Calendly account.

Before you begin

The Outlook Plug-in is for Windows only and is not available for Mac. Calendly's Outlook desktop plug-in works with:

• Outlook 2007 or later
• Windows XP or newer

Calendly's Outlook plug-in is not compatible with Outlook versions downloaded from the Microsoft/Windows store. Outlook must be downloaded from your O365 account in order to be compatible with the Outlook plug-in.

Then, check out this community article for more information on how to get it setup if it is the best option for you!

I hope this helps!