Heatmaps are primarily for visualizing distributions of values.
We expose the HEATMAP operation in the Calculate clause for columns that are numeric. If your dataset only have string values - you will not see the option for HEATMAP.
Note that if you find yourself in this situation by mistake, i.e. sending in numbers that are incorrectly identified as string fields, you can go to your dataset's Schema page and change the type of the column - e.g. from string to integer - and we'll coerce the column type correctly and immediately let you query on historical data.)