How to close menu in noindex menu using page definition via if else?

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Help plz. , the task is to close the main menu from indexing in on all pages except the main one. I display two menus through the condition, one with nofollow, the other without, I need to add the noindex attribute to the "mainnav2" menu, but the error constantly pops up, I don't know what to do anymore:
<?php if (is_front_page()) {wp_nav_menu ('menu= mainnav');} else {wp_nav_menu('menu=mainnav2');} ?>