I don't get it
You might be immature.
Lemme explain:
Levi Cohen is a British singer. He put out a debut album. In order to market said album, posters were posted (what else would be done with posters) all over Ultra Orthodox Right Wing Fundamentalist enclaves, such as Crown Heights. 770, named for its address on Eastern Parkway, is the headquarters of Chabad Lubavitch, and a highly popular destination. This poster was posted on a bulletin board outside one of the women's entrances.
The designer of said poster made use of "white space." White space is not an anti-safe space or Nazi gathering, it is kind of a no man's land which counter-intuitively draws attention to the non white space on the posted poster.
A visitor to 770 decided to make use of the white space on Levi Cohen's posted poster to share a message. For the sake of this explanation, I'm going to make an assumption that this was a woman, based on the hand writing and the location of the posted poster. This woman is apparently extremely frustrated that people bring "shaymos" to 770. There are people who will remove shaimos for you, but they charge. This woman finds it irresponsible and immature for people to leave items belonging in shaimos in 770 instead of paying for its burial. She therefore warns that such people dare not do so, and implores them to take responsibility and put an end to their immature behavior.
It is entirely possible that this woman was "subtweeting" a specific individual, and not calling out many people. It is also possible that her desire for people to bear responsibility and behave maturely was intended more broadly than only shaimos littering.