Unable To Create an Account Engagement Sandbox Business Unit in a Salesforce Sandbox Org

The root cause as to why the Business Unit Setup page in the Sandbox Org does not show the option to create a new Account Engagement Business Unit is usually due to one of two reasons:

1. Account Engagement Edition Type Does Not Support Account Engagement Sandbox Feature - The Account Engagement Sandbox feature is available by default only with the Account Engagement edition types of 'Advanced' and 'Premium' editions. It is not available in the Account Engagement 'Growth' edition or any legacy Account Engagement edition. If you have the 'Plus' edition, it's only available if purchased as an add-on and without this purchased, it would not be available in Plus edition.
2. Salesforce Sandbox Org Was Created Prior to Having an Account Engagement Edition Type Eligible For the Account Engagement Sandbox - If you have an eligible Account Engagement edition type of 'Advanced'/'Premium' or have purchased it as an add-on with 'Plus' edition but still can't create a new Business Unit in a Salesforce Sandbox Org, it's possible the root cause of this is the licenses purchased in production are not up-to-date in the Salesforce Sandbox Org you're encountering the issue in. To resolve this issue, you would have to use the Match Production Licenses tool within the Salesforce Sandbox Org via the steps in this article.