"Sandin, great work, and happy holidays!"
Sandin is the subject, a person.

Is the sentence above correct?

My family, has told me countless times that I need to improve my grammar, that is what I am here to do.

Also if there are any errors in this post let me know, thanks.

It is difficult to tell without context.

It reads like a list of three items.

If this is said by someone saying to Sandin that he had worked well and wishing him happy holidays, I would punctuate it differently:

"Sandin! Great work! And happy holidays!"

No problems with your post, except for a superfluous comma after family.

NB