Dear Wafaa,
First at all, I advise you to take a look at the documentation (Validation Manuals, User Manuals and Examples found into v7p2r1 folder), it is a very good way for beginners to learn how to run a model.
According to the error you got, I think what Christophe tried to tell is that you were executing the program from the wrong command window. A different one than the one you executed the compilation from.
A Step-by-Step for running any simulation:
1. Create the files required: steering, geometry, boundary conditions, among others (depending on your case).
2. Put them into the folder with the path C:\opentelemac-mascaret\v7p2r1\scripts\python27.
3. Run the command 'telemac2d.py file.cas' or 'python runcode.py telemac2d file.cas'. (file is a generic name, for instance if your steering file is oualidia.cas, that is what you have to put instead of file.cas).
It should start the simulation.
Now, as I know you used the Installation Guide I made, make the change to the shortcut (TELEMAC v7p2r1) on the field 'start in' as indicated in the attachment (image1). Once you have done it, just run the command (step3) as indicated in the attachment (image2).
Regarding to your steering file, it needs to be more specific, it lacks important parameters, such as initial conditions, prescribed values, time step, among others).
I hope have been clear enough.
Cheers,
José Andrés.