What does "pack" mean here? and how do you use it?

Can I say “don’t make mistake” instead of “make no mistake”?

Unfortunately, no. In this case, you can only use "make no mistake." "Make no mistake" is a common expression that is used to stress the truth or accuracy of a statement. In this video, the man is saying that the weed, Giant Salvinia ruins and kills the wildlife of the lake. "Don't make a mistake" means to avoid making errors while doing something. Even though these expressions sound similar, they have different meanings. Here are a few examples of "make no mistake" being used. Ex: Make no mistake, I will win this race. Ex: Make no mistake, this is going to be a great year.

What's he mean by "reputation" and when's it used like this?

He means a certain image and way people perceive him. Perhaps due to his behavior, actions, or characteristics. This can be used if something or someone is known for something - they have a "reputation." Usually in the negative sense, or one that isn't true to who someone is. "Rep" is also an informal shortened form of "reputation" that is often used. Ex: She's got a reputation for breaking people's hearts. Ex: You're giving this place a bad rep by bad-mouthing it. => "bad mouthing" means talking poorly of bad of something Ex: I have a reputation of being a good person to uphold by participating in all these charity events.

What's "well off" mean?

To be "well-off" means to enjoy good economic standing, to be wealthy or rich. It refers to someone who enjoys a very good standard of living. Ex: My friend Martha doesn't have a job. Her family is quite well-off and supports her financially. Ex: My family wasn't very well-off, so I had to support myself through school.

Just curious, what's the origin of the word "vaccine?" Is it a Latin word?

Vaccine and vaccination are terms that come from "Variolae vaccinae" (smallpox of the cow), the Latin term created by Edward Jenner. He was the scientist who developed both the concept of vaccines and created the first vaccine. He first used the phrase in 1798 for his research, so the term has been around for a very long time.

Is it common to say "I'm glad we're doing this." while dating?

It's a common phrase to say when hanging out or doing an activity with someone! So it could be used for dating, or with friends too. Like, if you've wanted to do something with them for a while, or weren't sure, then "I'm glad we're doing this" is a nice phrase to express your pleasure concerning hanging out or the activity at hand. Here, they're not even officially dating, he's expressing his interest in the walk they're on! Ex: I'm glad we're finally watching this movie together! EX: I'm so glad we're doing this. It's nice to have dinner at a friend's house.