Let’s get started by defining an object-specific action. We’ll create an action so that, for example, if your users open the Acme account, they can click on your New Opportunity action. The new opportunity they create will be automatically associated with the Acme account.

First, we need to find and open the Account object.

From Setup, enter Object Manager in the Quick Find box, then click Object Manager.
Enter Account in the Object Manager search box.
Click Account to open the object and then click Buttons, Links, and Actions.

Click New Action.
Select the type of action you want to create. For this example, let’s choose the Create a Record action.

In the above trailhead guidelines, I am not getting object manager when searching in quick find box.