The workflow is looking for a Date and Time field, but lookups return text. You need to return the field as Lookup text.
Set Variable: lookupStart to Current Item:Lookup: Sprint Start Date, Return fields as Lookup Value (as Text)
The workflow is looking for a Date and Time field, but lookups return text. You need to return the field as Lookup text.
Set Variable: lookupStart to Current Item:Lookup: Sprint Start Date, Return fields as Lookup Value (as Text)