The 50-0 pound-for-pound king takes on the YouTube sensation in Miami on Sunday evening.
But viewers in the United States were unable to watch the Chad Ochocinco vs Brian Maxwell bout that kicked off the undercard with the Showtime app malfunctioning as the card started.
“If you are having issues watching the fight on showtime.com or on the SHOWTIME app, please try again in 10 minutes. We are working to resolve the issue,” a short Showtime statement read.
The pay-per-view price in the US is $49.99 leaving many who had paid demanding a refund.
“That’s 10 mins of the fight we’re missing. I want a refund!,” one social media user said.
Another added: “I want a refund too.”
A third said: “I’m having an issue. Show is already on too. We deserve something for this.”
Mayweather and Paul meet in an eight-round exhibition fight to headline the card in Miami.
Social media star Paul, 0-1 as a professional boxer, has been full of confidence in the run up and believes he will shock the world.
Mayweather, 35 pounds lighter than his opponent on Sunday evening, is not so sure.
“I’ve been here before, so I know what it takes,” he said ahead of the fight. “I’ve fought every different style you can possibly fight.
“Height don’t win fights, weight don’t win fights, fighting wins fights at the end of the day and I can fight.
“Once again, I’m not worried about a hat. I don’t worry about what nobody says. The one thing I can do, I can fight. I’ve been at the top level for 25 years, so I know what it takes when it’s a fight of this magnitude.”