Some people have been having trouble activating latest Windows 10 builds. This is probably because Microsoft have changed the product keys that are used in the Insider Preview, beginning with Build 10158.
If you do a build-to-build upgrade through Windows Update, you should not need to enter a product key. But if you are prompted for a product key on Build 10158 or a later build, the new keys are:
For Windows 10 Insider Preview Home Editions: KTNPV-KTRK4-3RRR8-39X6W-W44T3
For Windows 10 Insider Preview Pro Editions: 8N67H-M3CY9-QT7C4-2TR7M-TXYCV
For Windows 10 Insider Preview Enterprise Editions: CKFK9-QNGF2-D34FM-99QX2-8XC4K
It should also be noted that beginning in Build 10158, clean installs of Windows 10 Insider Preview will prompt you to enter the appropriate product key during setup to activate Windows.