This issue did not exist in versions before 11.4, due to the default behavior of serial data loading where the "Get Values" button is enabled on the second prompt to select values as the first table is already loaded.
Starting from version 11.4, this is a desired behavior for parallel loading (introduced in that version) due to which when we are in the second prompt dialog for the second table, the first table has not been loaded yet. Hence, the "Get Values" button is disabled.
To overcome this in versions 11.4 and later, it is required to disable the parallel loading as described in the following Knowledge base article:
https://support.tibco.com/s/article/How-to-turn-off-parallel-data-loading-in-TIBCO-Spotfire