I’m (still) a goofy and immature person, but
It is usually used in the sentence “ふつつか者ですが、どうぞよろしくお願いします” meaning “Although I’m still a goofy and immature person, please get along well with me”. It is a lovely expression to humble oneself, which is typically used in manga/anime when one accepts someone’s love confession to him/her (but in reality, a lot of people find this phrase too cute to use). Some people also use it when they introduce themselves in a new community, or when they post something on social media on a special occasion (e.g. their birthday) to show their willingness to stay connected with their friends.
It’s also written as 不束者 in kanji, but not commonly used.
See the post below.