Error 13542 enabling system-versioning for existing tables
I had similar issue and splitting alter table into two separate statements solved it. Try

ALTER TABLE dbo.ClientBeacon ADD 
  datetime2(0) GENERATED ALWAYS AS ROW START HIDDEN CONSTRAINT F_ClientBeacon_SysStartTime DEFAULT SYSUTCDATETIME(),
  SysEndTime datetime2(0) GENERATED ALWAYS AS ROW END HIDDEN CONSTRAINT DF_ClientBeacon_SysEndTime DEFAULT CONVERT(datetime2 (0), '9999-12-31 23:59:59');

GO

ALTER TABLE dbo.ClientBeacon ADD 
  PERIOD FOR SYSTEM_TIME (SysStartTime, SysEndTime);

GO