If you're using MATLAB R2020b or later, use the subplot function.
If you're using MATLAB prior to that release, you could add a second row to your title using
title({'main','secondary'})
Your error message mentioned "suptitle" suptitle used to be a function in the Bioinformatics Toolbox. It created a super-title over a group of subplot. That can be acheived using in modern Matlab by adding the title to a TiledChartLayout object. Also see example in the documentation.
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