Most people can make use of external condoms without issues, but they may not be the most effective method of contraception for others.
Some people suffer allergic reactions from latex condoms. If this is an issue, plastic condoms made with polyurethane or polyisoprene are less likely to cause allergic reactions.
If you suffer from erectile dysfunction and are unable to maintain an erection, then you might be unable to use a condom as they require your penis to be fully erect to prevent the condom from slipping off.