I want to know how do i pass a variable as a argument in a function

df[col_name] not in range(18,121) was wrong, as @Panagiotis Kanavos pointed out. A simpler method if you're not df['single'].between 18 and 120.all():`

In addition, if df['single'].isnull(): was the wrong value. .any() can be utilized.

Snippet:

df, meta = pyreadstat.read_sav("SL_2025.sav")

def single_check(single):
    if df['single'].isnull().any():
        print("Missing values")
    
    if not df['single'].between(18, 120).all():
        print("Range error")
    else:
        print("Data is correct")

You can pass a variable as a argument in a function.

single_check(df['hCurrAge'])