How to resolve errors in the simulation design specifications for distillation columns?

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An error was encountered in the simulation design of the distillation column: Block: B1 Model: RADFRAC Convergence iterations: OL ML IL Err/Tol 1 1 4 26.827 2 9 33 29.406 3 4 11 18.654 4 3 7 1.3005 5 2 4 0.75272E-01 ** ERROR: The design specifications are not satisfied because one or more manipulated variables are at their limits. By searching on forums, it was found that many simulations encountered similar errors. Following the suggestions provided on those forums, the range of the design variables was adjusted, the number of trays was changed, and the design variable D/F was replaced with Distillate rate; the algorithm was also changed to Sum-Rates. Yet the simulation still produced errors. Could someone experienced in this field please explain what might be causing this issue? At the same time, I’ve seen many requests for help regarding this kind of error on the forum; could any friend summarize the general solutions to such errors? Since the bkp file cannot be uploaded, you can get in touch with me!