sorry it took so long to answer. Unfortunately I could not completely reconstruct your error. When using the same simulation parameters as you do, simulation results actually show some BLER. However, I have a few hints and questions for you.
Plase note, that when using multisim, parameters are taken from the parameter file specified in the corresponding multisim config file. In your case, if you are using the doc_mimo.m multisim script, this is the load_params_doc_antenna.m in the examples folder. This means that you have to configure all simulation parameters here rather than in LTE_UL_load_parameters. Further, I highly recommend to change some of your simulation parameters. First, it seems you simulate only 2 points of SNR which will not give you nice curves as result. I recommend to simulate 8 points of SNR spaced from 0 to 30dB. Second, if you are interested in BLER, you should increase the number of subframes simulated. In your current setting, you only use 100 subframes, meaning the lowest BLER you are able to simulate is 1/100. Also, I advise to reduce the simulation bandwidth to reduce the simulation time. Since the number of transmitted symbols grows with the bandwidth, also the simulation time increases super linear with the bandwidth. To obtain initial results I advise you to choose the lowest bandwidth of 1.4MHz. Especially when simulating MIMO, a high bandwidth will lead to extremely long simulation times (maybe in the order of many days) which is probably the reason why your MIMO simulation is not finishing.
Which version of the simulator are you using? Is there any reason to use 20MHz bandwidth?