GST_Applied is a Yes/No data type so don't use apostrophe delimiters for Boolean constant.
That expression is not adding anything.
GST: [Item_Amount]*[Item_Price]*IIf([GST_Applied]=Yes, 0.125, 0)
Then need to add GST to cost.
Total: [Item_Amount]*[Item_Price] + GST
Your table relationships have circular references and this is carrying over to query design. Fix the query to eliminate circular links.
Review
https://www.codeproject.com/articles...atabase-design
Suggest remove GST_Application and Supplier links to Procurement.
You want to calculate the Total of all Items being ordered for what - one supplier? Main Page is bound to Users table, so calculations on ordered items will likely involve DSum() Domain Aggregate function.
Need to calculate a field that assigns fiscal year to each record. Then that field can be used for filtering records by fiscal year. And I see you are calculating FY from the year of Indent_Date - why, what is this date for?
I am not entirely clear what you want the combobox to list. Perhaps provide a sample here in post. Use Advanced post editor to build a table.
Strongly advise not to use spaces in naming convention.