uCheck変数にcmを入力すると、プログラムはelse条件で文字列を出力します
uCheck=input(print('Unit?')) #<--I enter 'cm' here
if uCheck=='nm':
Wave=Wave
if uCheck=='cm': #<--it seems to skip this boolean
Wave=Wave*(10**7)
if uCheck=='mm':
Wave=Wave*(10**6)
if uCheck=='m':
Wave=Wave*(10**9)
else:
print('Invalid Unit! Valid units are: nm, mm, cm, m.') #<-- and prints this
Frequency()