Disclaimer: I use Proseries
It is available in ProSeries so I'm betting it is available in Lacerte. To activate in ProSeries there is a check box on the Federal Information Sheet - then when printing the 1040 prints as 1040-SR. There is not an actual 1040-SR form on the screen.
It is available in LC
Check your settings so that you allow the SR to be printed.
Also, Input Screen 3. See Screenshot below.
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