Shared topology creates a fluid-structure interface, there is no need for contact.
For the current version of Ansys 2023 R1, in Modal Acoustics, define an Acoustic Region (the fluid body) and a Structural Region, (the container). That causes elements to have the correct element types assigned. When you solve, look at the text in the Solution Information folder Solution Output to see what elements types were created. You should see FLUID221 elements in the list.
In Ansys version 18.1, look in the Toolbox in Workbench, you will not find Modal Acoustics there. Use a Modal analysis. In Mechanical, use a command object under the fluid body in Geometry to assign the element type.
I don’t know if a fluid Free Surface is supported in Ansys 18.1 for acoustics.
Do you have a second computer where you can install the free ANSYS Student 2023 R1 software? Then you will have an easier time getting help. There is a risk of messing up the licensing for Ansys 18.1 if you install the free Ansys Student 2023 R1 on the same computer.