Hello
The Classic Outlook is a paid app, that's part of the Microsoft 365 subscription.
Can you please confirm if you have Microsoft 365 personal/family/premium? If so, if you install the entire office apps, it should include Classic Outlook. Intallation instruction is here:
Please go to https://www.office.com
Sign into the Microsoft account you used for the subscription. click on Apps on the left.
Then on the upper right, click on "Install apps". This will now download the installer file. Once downloaded, click on the file to install. It will install the Desktop version of the apps for you then.
If you have installed it but switched to the new Outlook and need classic back, you can download it here https://support.microsoft.com/en-us/office/install-or-reinstall-classic-outlook-on-a-windows-pc-5c94902b-31a5-4274-abb0-b07f4661edf5