'STRING_SPLIT' is not a recognized built-in function name

It was a syntax error. I was attempting to use the function as scalar and not as table valued.

Correct Syntax: SELECT Value FROM STRING_SPLIT('Lorem ipsum dolor sit amet.', ' ');

The STRING_SPLIT function is available at compatibility level 130 or higher. If your database compatibility level is lower than 130, SQL Server will not be able to find and execute STRING_SPLIT function. You can change a compatibility level of database using the following command:

ALTER DATABASE DatabaseName SET COMPATIBILITY_LEVEL = 130

EDIT: Note that compatibility level 150 might be default even in new Azure SQL Databases.

For reference:

Version - Highest Compatibility Level - Lowest Available Level

• SQL 2019 - 150 - 100
• SQL 2017 - 140 - 100
• SQL 2016 - 130 - 100
• SQL 2014 - 120 - 100
• SQL 2012 - 110 - 90
• SQL 2008 - 100 - 80
• SQL 2005 - 90 - 80
• SQL 2000 - 80 - 80