User is missing the Account Engagement Lightning App tile from the App Launcher

1) Missing permission set:
An admin will need to assign the permission set to any users who need to access the Account Engagement Lightning App tile in the App Launcher:

1. From Marketing Setup, in the Quick Find box, enter Permission Sets, then select Account Engagement User, Sales Cloud User, Service Cloud User, or CRM User from the list.
2. Click Manage Assignments.
3. Click Add Assignments and choose all users who need access to the Account Engagement Lightning app.
See this article for further information on assigning permission sets.

2) Not enabled in the App Menu:
▸ In Salesforce, go to Setup > in the ‘Quick Find’ box > search for “App Menu”.
▸ Look for the “Account Engagement” app
▸ Ensure this displays “Visible in App Launcher” instead of “Hidden in App Launcher”.