Depending on where this expression is being used, it appears you need to adjust how you are declaring your fields.
If this is being used in a matrix or table with an explicit dataset assigned, you don't need to specify the dataset.
=Fields!Bathroom.Value & " Bathroom " & Fields!Bedroom.Value & " Bedroom"
If this is being used in a textbox, you'll need to specify some function to use with it, such as the First
function, which will give the first item from that column.
=First(Fields!Bathroom.Value, "PropertyDetails") & " Bathroom " & First(Fields!Bedroom.Value, "PropertyDetails") & " Bedroom"