bro, i had problems again,
why i couldn't make inlet face on flow simulation,
see my picture to see my problems
I dont know how is the part looks like but generally you should pick the face which is inside of the part. Not outside face like it is at your picture. You cannot do flow simulation in open space therefore outside face cannot be the one in contact with the fluid.
If you want to make flow simulation of the air flowing around this part (for example) you should make one bigger part (i.e. pipe) and your tested part should be inside of the pipe. After that you have to close the pipe from both ends and pick faces of these ends which are inside of the pipe.
file is missing. Anyway - lets presume that you want to see how the air will flow around your part (it looks like some cube). You have to make an assembly and place this cube in some bigger part. Lets make empty pipe and then place the cube INSIDE of the pipe. After that make lid (cap) on both sides of the pipe. Dont MERGE these lids together with the pipe.
When you are ready with this use "Section View" along the length of the pipe so that you can see inner faces of these lids (these will be your inlet and outlet faces) and you also should see your cube. Use MATES to place your cube. Note - the pipe has to be fully closed. Pick the flow simulation and that's it. This is very basic way how to do it. No materials, temperatures, liquid type and other stuff was selected. You also can use Youtube and find step-by-step videos made by "GoEngineers". Good luck.