That doesn't tell much? Where are you getting Lastpass from? And what does the error screenshot says? This way others reading this can see or identify what is going on with the install.

cannot add lastpass add-on to firefox

Are you referring to Lastpass at https://addons.mozilla.org/firefox/addon/lastpass-password-manager/ ?

If so then the most recent version seems to have some issues based on the Reviews so perhaps you could try installing the previous version? on https://addons.mozilla.org/firefox/addon/lastpass-password-manager/versions/

You can try to uninstall and reinstall LastPass as that seems to work in some case (could be an issue with broken extension storage).