This is not OK! NG stands for "No Good" and is used as the colloquial antonym of "OK" in Japan. Chat partner wants to save you from a mistake or warn you about something.
The NG button emoji was introduced in 2010 and released as part of the Unicode 11.0 standard. This emoji can be found under the symbols
category. Because the NG button emoji has been around for quite some time, it should display correctly in the majority of operating systems.