SSRS - How to hide the "ERROR: subreport could not be shown"

Problem:
ERROR: subreport could not be shown
shows up wherever the subreport doesn't get valid parameters

By default, SSRS will not render an empty report.

The error you are getting is due to the fact that the parameter you are passing to the subreport is an invalid parameter because you have setup Parameters to be taken from a query, thus limiting the list of valid parameters.

If you change the available values of the subreport parameter to "none" then the subreport will accept all inputs, and simply render blank instead of throwing the error.

Solution:
Change the "Available Values" option of your subreport parameter(s) to "none"