This?
In[1]:= Assuming[{s > 0},
Integrate[Exp[-(x - xbar)^2/(2 s^2)], {x, -Infinity, Infinity}]]
Out[1]= Sqrt[2 [Pi]] s
Note: Using > implies, without really being obvious about it, that this also means "element of the Reals."
I am assuming, based on the error message immediately before your Integral, that you have fallen into the "I must desktop publish my Mathematica input" trap and that everything, no matter how desktop published it is, can be expected to work like any other simple ordinary variable. That trap bites people again and again and again and ... and does not seem to be able to be resisted.
