CHOOSE function not working in trigger

I have a field called RESPONSIBILITY which is an integer and will hold a value of 1-3. I'm trying to populate a table using the CHOOSE function and it's rejecting it as not a built-in function, even though it shows as a built-in function when I hover over it with the mouse. I'd love to attach a screen print, but it won't let me. Here's the statement:

Insertinto xxxxx.PROJECT_MILESTONE select LEFT(a.pk,13) + RIGHT(b.SEQUENCE,3), a.PK, a.PROJECT_NAME, b.PK, b.Sequence, b.DESCRIPTION, null, b.BUDGET_COMPLETE_DAYS, null, null, null, null, null, null, CHOOSE(b.RESPONSIBILITY,a.OWNER_ID,a.PROVIDER_ID,' ') from dbo.project a, dbo.program_milestone b where a.FK_PROGRAM = b.FK_PROGRAM