Master is OH3 now. Because of some differences (e.g. namespace changes) OH2 bindings won‘t work for OH3 (they will be ported later). Since there is no working OH3 runtime at the moment, 2.5.x would be a good choice if you want to get it running ASAP.
@Bruce_Osborne I appreciate your dedication, but your last to comments are unrelated to the issue mentioned here. The original topic is related to changes of name space changes and affects current development, not running openHAB itself. Please be careful when answering topics in the development category. To give good answers questions in the development category it mostly require knowledge about the development side of openHAB.