И выражение должно быть из реальных столбцов модели.
How RANKX works
The syntax for RANKX is as follows:
RANKX(<table>, <expression>[, <value>[, <order>[, <ties>]]])
<table>
The first parameter <table> needs to be a DAX table. This can be a physical table, or it can be the output of a DAX function that returns a table. Like any of the X functions (SUMX, MINX, MAXX etc), the table that gets passed here controls the number of loops, or iterations performed by the RANKX function. If we pass our sample 10-row dataset to the function, the code inside the RANKX function will loop 10 times (once for each row in the table). The smallest number that could be generated is a 1, while the largest value that could be returned is a 10. If we used a function to summarize the physical table, say to the distinct values of the Category column, the RANKX function will logically perform three loops.