Not enough time is a huuugggeee one and thinking exercise is boring is another one. There is also age limits or thinking exercising is too expensive.
Too expensive- You don't have to just work out at a gym or buy a bunch of fancy equipment. There are plenty of free ways to exercise: do strengthening exercises at home (usually what I do because I dont want to pay for a membership) There are also free and fun exercise classes that most communities do at your local recreational center or gyms.
Exercise is boring- Exercise is not boring if you find something you enjoy. Start a work out with a friend or listen to your favorite music to get you motivated. Choose different activities, there are sooo many different activities you can do other than push ups or running.
Main thing- Think about the results in the end and keep pushing!
Hope this helps, if you have other questions let me know :)