You may occasionally receive the following message when clicking on your Linktree bio link from your Instagram profile:
This message is a security measure from Instagram itself which we have no control over, unfortunately. It is randomized and can happen to anyone - even non-Linktree users. It is put in place by Instagram to check the links in your bio and make sure they’re not a security risk.
Simply quit out of Instagram and click the link again - it won’t appear anymore. Now Instagram recognizes that you and also your links are not spam, it shouldn't ask again.