Is there a regulation that states a Soldier cannot go to a school 60 days from a PCS?

Though I won't be much help on your specific question, I just want to congratulate you on your advancement from enlisted to officer. In my time, some of the absolute best officers I met and served with and under were Mustangs. The key to their success was that they never forget their roots.

TDY enroute?

Likely your old unit just doesn't want to pay for your slot if you won't be around afterwards to use the training you received there.