John Gill Commentary Deuteronomy 4:15

"Take ye therefore good heed unto yourselves; for ye saw no manner of form on the day that Jehovah spake unto you in Horeb out of the midst of the fire." — Deuteronomy 4:15 (ASV)

Take you therefore good heed to yourselves
As to keep all the laws given them, so particularly to avoid idolatry:

for you saw no manner of similitude on the day the Lord spoke to you
in Horeb out of the midst of the fire ;
and therefore, as they had nothing that directed and led them, so they had nothing that could be a temptation to them, to make any form or likeness, and worship it.