Why am I getting the below error related to ST parameter missing?
The error screenshot suggests that the seismic definition is not having any ST parameter. ST parameter is used to define the structure type which is used to calculate natural period as per clause 7.6.2 of IS1893 2016 part 1. So here you need to go to seismic definition and double click on the definition to open the window. You will find that the ST parameter value is blank and you need to pick the correct one from the dropdown as per your requirement. You may also edit it directly from input editor if you are familiar with the syntax.