I have seen this problem on Windows 8 and 10. It is a real problem and most time prevents you from completing the installation. Not just LastPass but other applications as well. https://helpdesk.lastpass.com/wp-content/uploads/installer_import_win.png
In this case, there is a button that needs to be pressed to either "Import" or say "No Thanks" to the dialog window. Unfortunately the programmer at LastPass did not include hot-keys or a good cursor navigation in their efforts. I have found by trial an error through a combination of changing window focus by selected another window and then coming back followed by a tab or two will eventually let you move forward. I can be certain if I imported or said no thanks but was able to get passed the issue.
Come on guys at LastPass. Please add a hot-key like "Alt-I" to select Import or "Alt-N" to say no thanks. This is a simple best practice. Also, please enable the ability to re-size the window. Please!!!