I am designing a concrete beam and getting a message A suitable bar arrangement could not be determined. What does that mean ?

After the required reinforcement is calculated, the software finds out how many bars of what sizes are required to satisfy that reinforcement requirement. It then tries to arrange the bars at the different faces and checks whether code stipulated maximum or minimum spacing requirements are satisfied by the arrangement and also whether the code requirement regarding clear spacing between adjacent bars are met. If it fails to arrange the bars as per these criteria, it tries to come up with up with an acceptable arrangement by using a different sized bar which satisfies the minimum and maximum bar diameter limits set by the user.

If none of these trials with different bar sizes produce an acceptable bar arrangement, the software prints out a message letting the user know that a suitable bar arrangement could not be arrived at. In such cases users may play around with the minimum and maximum bar sizes and try to increase the range of bars that can be tried out. If that also does not work, using a section with a larger depth or larger width is the only option.