With the 26 letters of the alphabet, we can make pretty much any sound present in the majority of languages. But Japanese just doesn’t contain certain sounds present in English, like “th” or “v”, and their “r” is somewhere right between our “r” and “l”, making them sound almost exactly the same to Japanese ears.
Since most Japanese people grow up only speaking Japanese, it means that when they start learning English at school, they either have to learn entirely new sounds (difficult) or else try to render English in Japanese sounds (which isn’t accurate). As a result, many Japanese English learners feel a lot of anxiety over the accuracy of their pronunciation. But should that really be holding them back?