"Unable to BTO" error

Why did I get the following error message when attempting to buy a stock yesterday?

"Unable to BTO 62 VECO - Current account cash is $958.62; proposed trade requires cash of $984.25 (code=pre)."

It appears to indicate that I don’t meet the margin requirement. However, I calculated the margin requirement manually and it is nowhere near the limit.

At the time of the attempted transaction yesterday:

“Total System Equity” reported by C2 = $18134

Total market value of stocks in my account including attempted VECO purchase = $31512

18134 / 31512 = .576 = 57.6%

The C2 FAQ says that traders “need only to put up 50% cash for any given position”. 57.6% is well above the 50% requirement.

Because you have only $958 in cash and need $984 to make the trade.

I am still getting errors for "Unable to BTO".

Another forum post says that C2 initiates a margin call when buy power goes below zero. My buy power is $4688, so why am I getting an "Unable to BTO" error that prevents me from buying $984 of stock?