I'm trying to link my TD Ameritrade, and it is asking for Account Number 9 digits...but my account number is 8 digits. It's an old account back when it was Scottrade.

nevermind silly me.  i found it.

Call TD Ameritrade and let them know the issue and ask them what the fix is.

This is speculation but something tells me that since you have an old 8 digit account, and the current format is 9 digits that the answer will be to put a 0 in front of the first digit. (ex 12345678 = [number removed]).

Hope this helps