A few things:
Re:"I am getting error in it." and "But I got error while running."
Saying what error would be very helpful.

Re:"I put a MsgBox in the code  to catch the value. But it didn't show
anything."
This happens when there is nothing in the cell. Cells(7,2) refers to
cell 7 rows down and 2 columns across, ie. B7.

As your code stands it tries to add  (not concatenate) the following 4
things:

what it finds in row 7, "Y", what it finds in row 8, "M"

and it looks to row 2 to see how many columns it should process (I
imagine these are headers?). So as long as there's something in the
column in row 2 it tries to do the concatenating. When it reaches the
first empty cell in row 2 it stops looping.

Now all you need to do is change every instance of a '+' sign in the
code to a '&'. This should stop the Type mismatch error.

Cells(9, y).Value = Cells(7, y).Value & "Y" & Cells(8, y).Value & "M"

I note also that you have the line
x = 7
but never use x elsewhere.

I was using the + sign as I
got it in the code  from the Microsoft on-line training program. I am
pasting that code below for you. Here they are trying to concancate
the last name and first name, which are kept in two columns. Please
let me where am I going wrong here.

Sub LoopRange1()

'Have x start at row 3
x = 3

'Loop until a blank row is found
Do While Cells(x, 3).Value <> ""
'This will put the value of the third and fourth columns (C
and D) together,
'with a space between, and into the fifth column (E)
Cells(x, 5).Value = Cells(x, 3).Value + " " + Cells(x,
4).Value
'increase the value of x by 1 to act on the next row
x = x + 1
Loop

End Sub

This code  seems fine. The + sign is no problem if the things you are
trying to add  or concatenate are all of the same type. In the code
below they're all strings. If they were all numbers and you didn't
have the space in there it would add the numbers together instead.

The & sign is the vba  string concatenator. If you had numbers in there
and used the &, the numbers would be treated as strings and
concatrenated.