I lost my activation lock n how can I get?

What do you mean you lost your activation lock. It is not something you can lose, unless you are talking about not being able to open Activation Lock because you have forgotten your Apple ID.

What does "acting up" mean?

That means the activation lock was placed by someone else. If this was a previously owned phone the previous owner did not remove activation lock before giving it to you. They are the only one who can remove Activation Lock. This is what they should have done→What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support, and they can still do it, remotely if necessary.

See this user’s other post→https://discussions.apple.com/thread/252912108