Hi,
Some of the variables in OptiSystem are used by multiple components to calculate results. Bit rate is one of those! If you are a beginner the best choice is to change this global bit rate setting (along with the Symbol rat, which should equal your bit rate for NRZ modulation).
To change it globally double click anywhere on the layout and modify it from the Layout Parameters. In this particular case you modified the bit rate in the PRBS Generator, so the global value was not updated. The BER Analyzer is then expecting a different bit rate and time window then is actually being transmitted.
Cheers!