First you say [Date Created] is a text field storing value then you show [DateCreated] as a calculated value. One name has space, the other doesn't. Are these the same field?
SLA is a number, don't use apostrophe delimiters. Only one IIf needed.
SLA_Status: IIf(([Status]='30 Day Notification' And [SLA] <=16) Or ([Status]='15 Day Notification' And [SLA] Between 14 And 31) Or ([Status]='1 Day Expired Notification' And [SLA] Between 31 And 61) Or ([Status]='30 Day Expired Notification' And [SLA] Between 62 And 80), "Within SLA", "Out of SLA")