If you don't see the "Get Add-Ins" button in Outlook when trying to install Oliv, try out the following scenarios and steps to fix it.
Scenario 1: Optional Connected Experiences is disabled
Go to File -> Options --> General
Click the "Privacy Settings" button.
If the "Enable optional connected experiences" button is not checked, the 'Get Add-Ins' button (among other functions) will be greyed out. Check it.
Restart Outlook and the "Get Add-Ins" button should be visible now.
Scenario 2: Account connection is misconfigured
Sometimes, the Outlook account is connected via IMAP / POP3 instead of a direct connection with the Exchange server. You need to sign out and then sign in with your credentials to connect the account again.
To check the connection type:
Click the File tab. Click Account Settings, and then click Account Settings.
On the E-mail tab, the list of accounts indicates the type of each account.
It should like below. Check if your account is connected as "Microsoft Exchange" (the desired option).
If it is connected as IMAP/POP/MAPI, sign out and sign back in with your Exchange / Office 365 credentials directly.
Scenario 3: Button was removed from ribbon accidentally
Go to File -> Customize Ribbon
If you see "Get Add-ins" on the left, create a group first by clicking on "New Group" and then move the "Get Add-ins" button to the right side.
Scenario 4: Admin has disabled third-party Add-Ins
Ask your Admin to sign into Office 365 with an admin account. They can then navigate to Admin center -> Settings -> Services -> User owned Apps and Services as the following picture shows and must enable both the options.