D&D 5E [5e] Swashbuckler/Fighter makeshift tank

[Eladain]

This is my first dual wielder in 5e so forgive the lack of knowledge here. However, if booming blade uses your standard action (as would lightning lure I believe), which part of that prevents you from using your bonus action for the off hand attack?

 

[Kithas]

This is my first dual wielder in 5e so forgive the lack of knowledge here. However, if booming blade uses your standard action (as would lightning lure I believe), which part of that prevents you from using your bonus action for the off hand attack?

In order to use your offhand attack as a bonus action you must use the Attack action. Both booming blade and lightning lure use the 'cast a spell' action so you can't use your bonus action that way. You still have your bonus action to use, for example cunning action still works but two weapon fighting requires the use of the attack action.