The Structutral solution is a beta feature as of version 2023 R1. To use it, it must be enabled from Tool --> Options --> General Options. In the tree on the left side of the Options dialog box, select General --> Desktop Configuration and then click on Beta Options. Locate the Mechanical Structural Design checkbox from the list and turn it ON.
This should enalbe the Structural solution. Please refer to the AEDT Mechanical User Guide to check out other details on model building and setup in Structural.