Reasons you can't connect with someone on LinkedIn:

• The most typical reason is that their privacy settings are set to private.
• You must have successfully validated your account's primary email; you will not be able to connect to other accounts until this is done.
• You can check if you're already linked to that member by looking at the connection level; if it's first, you're already connected and can message them instead!
• You've already made a connection request to the member. You won't be able to send any more connection requests if the linkedin invitation isn't accepted.

Read More: Are connections and followers the same thing in LinkedIn?