You may occasionally receive the following message when clicking on your Linktree URL from within 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 - including 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, re-open it 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.