Because, you are trying to concatenate a value having different data types, (1) cmonth() function - date (2) val(year()) function - Val- is numerical function, while, year - is a date function. This will give you a operator/operand type mismatch error.

var1 = cmonth(date())
var2 = year(date())

n = var1+' '+alltrim(str(var2))

n = May 2005