You have to leave spaces in the squad if you can't meet the homegrown requirements. So, in your example, you're fine without Lawrence as although you've only got 5/8 players trained in Scotland registered, you have left 3 spaces in the squad of 25 (you've got 22 registered). Trying to register Lawrence, who is not homegrown, means you now only have 2 of the 3 spaces you need to leave and thus are not meeting the requirements.