I know exercising with a friend is a good way to stay motivated. However, sometimes my friend is not able to join me. How do you stay motivated when your friend is not able to join you?

Put your headphones on, select your favourite running/exercise playlist from youtube, and just go for it!
You will find that sometimes going solo really gives you ‘focus’ in whatever your aim is. A friend is great for motivation and also great for times for a much needed chat whilst on a walk or run, but it’s also a great strength to be able to go solo and find the real focus and concentration which can sometimes otherwise be lost in worldly chatter.