Try this
After selecting a block of text, press Shift+i or capital I.
Lowercase i will not work.
Then type the things you want and finally to apply it to all lines, press Esc twice.
If this doesn’t work…
Check if you have +visualextra enabled in your version of Vim.
You can do this by typing in :ver and scrolling through the list of features. (You might want to copy and paste it into a buffer and do incremental search because the format is odd.)
Enabling it is outside the scope of this question but I’m sure you can find it somewhere.