If you have a separate AdWords account, that could be one reason for running into this. I ran into this error in the promote tab, and was finally able to discover the cause and resolve it after some back & forth with the AdMob support team.
What was the cause?
I first joined AdWords a long time before I joined AdMob, and at the time the advice was to create a separate Google account specifically for AdWords and give "read only" access to the normal account. It turns out that this "read only" access causes the AdMob web console to go haywire and fail with "Internal Errors. Please try again" if you try to create a promotion through AdMob.
How do I fix it?
The fix is actually quite simple once you know what the problem is -- all it requires is signing into AdWords and changing the access on your AdMob account to "standard", since AdMob actually works through AdWords to create these promotions. House ads & other promotions should then work from that point on.
Now if only the AdMob team could improve their error messages for this situation, as that would have saved both me and the support team a lot of grief! ;)