make sure they match-e.g lowercase and no spaces.
Also, try to a calculated field for the parameter to the filter panel and type dayfixed and click the + button to add in the custom value list.
If this does not work, then it has to do with the data source
I have the same problem. It seems that when you put the parameter in, it is a string value stored in 'parenthesis'.
... WHERE <Parameters.FY/CY>=2018 .... (parameter can have value of 'fy' or 'cy' for switching between calendar or fiscal year)
... WHERE 'fy'=2018 ...
which will return error since you cannot compare string value to integer and even if you could, 'fy'='2018' retuns no values since they are not equal.
The correct way should be:
... WHERE fy=2018 ...
Unfortunately I don't know the solution to this, I suppose we could start an idea to implement this in Tableau (e.g. "Parameter defining field/table names or in Custom SQL") if it is not possible.
It should have nothing to do with the data source.