Hello Unity community!
This post assumes you have read and seen the two posts:
In 2021.1, the UNet HLAPI package has become a Deprecated package. This means it is officially unsupported for that version of Unity and onwards. User of 2021.1 cannot install the HLAPI package anymore via the Package Manager editor interface nor search for it by name.
In 2021.2.0b4, HLAPI has been removed from Package Manager registry. For information on what are suggestions are for substitute solutions, please see this forum post that includes a FAQ of commonly asked questions. If you have any additional questions, we are happy to address them if you post here to the UNet sub-forum.