The error message should actually refer to line B4, not B1.
The amount on line B4 is the gross amount of the distribution minus the greater of what you indicated the RMD and the QCD to be. (The amount on line B4 is also reduced by any amount rolled over as indicated on line B1 or B2, but TurboTax no longer support reporting a rollover and a Roth conversion together on a single 1099-R form, so there should be nothing on line B1 or B2).
