Rollups return an array, not a string. The FIND()
formula function looks for the value you pass in a string, but it doesn’t know what to do with the array you are giving it ({Deals Rollup}
).
You should be able to fix this easily by coercing the array into a string by just concatenating it with an empty string, like this:
FIND("....", {Deals Rollup} & "")
The & ""
will force the array returned by {Deals Rollup}
into behaving like a string, and thus play nicely with the FIND()
function.