IF/Case Statement Issue

I have the following fields
1.  SO TYPE (Multiple Choice)
2.  DATE FIELDS (Date)
3.  INITIATION DAYS (Formula Duration)
4.  INITIATION DAYS RANGES (Formula Duration)

My Initiation Days formula says the following and is working great!

If([SO Type]="R" or [SO Type]="LL", ([Portfolio Sent to USACE]-[Sent to QAE]),([Sent NTE Request]-[Date S/O Initiated]))

I am trying to write the formula for my Initiation Days Ranges formula with the following:

If([Initiation Days]<19,"0-18",
If([[Initiation Days]]>18 and [Initiation Days]<38,"19-37"),
If([Initiation Days]>37 and [Initiation Days]<57,"38-56"),
"More Than 56 Days")

But I am getting the following error:

'<' can't be applied on types duration, number

Can someone help me figure this out so that I can get this working!!!!!  It is absolutely driving me crazy.