It all depends on the individual and web-app type along its UI/UX.
Front-end is a bit easier to start(because you only need a text editor and browser), but hard to proceed(it requires a lot of extra learnings to be good in front-end side).
Back-end is relatively harder to start(you need to set up local environments, which is easy though), but easier to proceed(relatively fewer techs to learn).
In general for beginners, I find back-end development(a server-side programming language + Framework + SQL/No-SQL) to be easier than front-end(HTML, CSS, JavaScript + a Front-end framework + GraphQL(optional)). You can check this article on Backend to understand what skills are exactly required.
Additional to required technologies, back-end development requires you to have basic understandings of server platforms(operating system), while front-end requires basics understandings of client platforms(browsers and devices). While understanding of network is common in both. Advanced cases in each stack have their own sets of requirements and difficulties.