Trouple with Tip calculator

Whenver a user enters a number of users to be less than one or the bill amount to be less than zero , I want their to be an alert box saying “please enter positive values”. The ony thing is , my tip calculator calculates a value, even when It enters number of people to be less than zero or the meal amount to be less than zero. https://codepen.io/noblegas/pen/povPrBW

Anyone have trouble accessing my link?

For my Tip calculator, I don’t want my the word dollars to show yet. I only want the words “dollars” to show after I clicked the “Calculate” Button. When I write the line of code "
totalUnits.style.display=’ ‘;
actualUnits.style.display=’ ';", the words dollars do not disappear. https://codepen.io/noblegas/pen/povPrBW