Temperature based automation

Your screen grab is helpful, but it doesn't show the action ("send message") you mention.

Click on the third action 'If Temperature is less ..' so that it has a highlight around it (usually a blue outline).

Now in the window pane on the right, click the top left Action Library icon and find the action you need to happen when the temperature is low. Its a good idea when developing Shortcuts to simply put a message on screen, so choose Show Alert. This will be added as the next action below your highlighted action and will be aligned slightly to the right to show that it depends on the previous action (ie. it will not happen if the previous action is false). Triple click the default text in the Show Alert and insert your own words.

You seem to have deleted the Otherwise action which followed the If action.

Again, it's a good idea to leave that in whilst you are building the shortcut and put a different Show Alert in there with some alternative text to demonstrate when that branch is taken.

You need to get the Actions within the 'If' and 'Otherwise' sections.

Yours will always trigger because its after the 'End If'