=LAMBDA(parameter1, [parameter2, ...], calculation) defines a function. On its own in a cell it returns #CALC!, because nothing calls it. To try it, put the arguments in brackets straight after it:
=LAMBDA(price, cost, (price-cost)/price)(B2, C2)
The useful step is saving it under a name in Formulas > Name Manager > New: name MARGIN, and in Refers to the LAMBDA without the brackets. From then on =MARGIN(B2,C2) works anywhere in the workbook, like a built-in function, and a fix to the definition updates every cell that uses it.
Parameters follow the same rules as LET names: words, not cell addresses.