SQL Server: Invalid object name STRING SPLIT.

It seems your SQL Server compatibility level is incorrect.

First you can check what it is

SELECT compatibility_level FROM sys.databases 
WHERE name = 'ASPSnippets'

Then you can set it to 130. In order to use the function.

ALTER DATABASE [ASPSnippets] 
SET COMPATIBILITY_LEVEL = 130

SELECT compatibility_level FROM sys.databases 
WHERE name = 'ASPSnippets'

After that when you try again, it will work.