First of all, you tried to replace the entire 12.00 with '', which isn't going to give your desired results.
Second you are trying to do replace directly on a decimal. Replace must be performed on a string, so you have to CAST.
There are many ways to get your desired results, but this replace would have worked (assuming your column name is "height":
REPLACE(CAST(height as varchar(31)),'.00','')
EDIT:
This script works:
DECLARE @Height decimal(6,2);
SET @Height = 12.00;
SELECT @Height, REPLACE(CAST(@Height AS varchar(31)),'.00','');