Is it correct to say “i highly appreciate it” ?

Sure, it's fine. (UK)

I would say “I really appreciate it”

Personally I would go with greatly rather than highly, but there is nothing wrong with the highly.

I say that all the time, yes

It’s technically correct but it’s more common and natural to use “greatly” instead

Should be fine, but I will say "greatly appreciate it" does feel a bit more natural

Here in Appalachia in the US that's how we normally say it

“I really appreciate it” or even “I very much appreciate it” sound more standard

It is not incorrect, but the word 'highly' is VERY VERY over-used, especially in phrases like 'highly recommend'. I would say 'I very much appreciate it' instead.

just say “i really appreciate it.” using highly is very formal.

I believe, It is absolutely correct

Yeah that’s fine, it might be more common to say “much appreciated” but that probably depends on the region

Sounds fine to me...

It’s correct, but it sounds formal and stilted. “I really appreciate it” is more common and casual.

i'm not sure if it's correct but "i appreciate that a lot" sounds more natural

A stronger collocation is "greatly appreciate it" while "highly recommend it" would also be a stronger collocation.

England - I would say greatly, rather than highly. Highly also works though!

In my experience (USA) we’d say, “ I greatly appreciate ___.”

Perfectly normal. I use it in emails all the time.

It is correct but could be too formal based on the context. Substitute the word "highly" for "really" or other synonyms.

it is correct yes. if i were replying to an email and thanking someone i might write “thank you for xyz, it is greatly appreciated/much appreciated”. “highly appreciated” can also be used, for me it adds an additional level of appreciation that i would see as unnecessary unless you’re really really thankful for something.

Appreciate is a verb. Verbs require different adverbs to adjectives. You wouldn’t say “I very like”. Highly is a synonym for very. It is used with adjectives. You could say “I’m highly appreciative of …” But “I really appreciate…”

I'll generally say "greatly" instead.

Sure why not

No. But you can say "I highly appreciate it."

Yes.

Yes. “(It’s) Highly/much appreciated” also works.

It's probably correct but it's not common. Instead you can say: I really appreciate it.

Yes, but a more natural and relaxed way would be "I really appreciate it", or "I greatly appreciate it" for more formal

yeah, nobody really says that. its not wrong though– it gets the point across the same as "I really appreciate it" or "I very much appreciate it"

That's weird. I really appreciate it. I truly appreciate. I definitely appreciate it. But "highly"? I don't think that's a thing.