I'm totally unfamiliar with MS SQLserver, and PHP is not a language I have a lot to do with. But I'm intimately familiar with TurnKey, so let me have a crack at getting you going in the right direction! :)
Firstly TBH, I'm not sure whether your support question is a good fit for a Q&A site like this (at least not without much more info about the customisations that you've made and the steps previous to the issue where you find yourself). Regardless, I'll have a crack.
It's probably also worth noting that just in case it's nor clear, your current server is TurnKey v15.x - based on Debian 9/Stretch.
A quick google bought me to the MS docs which note there are some prerequisites. I'm not sure if these are related to your issue, but to ensure we're on the same page, please ensure that you have followed the relevant steps for Debian. I.e. as noted in these links:
You also didn't note how/where you installed PHP 7.4 from? I'm assuming that you installed via https://deb.sury.org/ i.e. followed the Debian steps here: https://packages.sury.org/php/README.txt
Assuming so, then I also suggest making sure that you have the latest version of PEAR installed (i.e. the one from Ondřej Surý's repo and not the one from Debian). If you want to check, try this:
apt policy php-pear
The Debian Streth version is currently 1.10.1 (full string: "1:1.10.1+submodules+notgz-9+deb9u1"). From what I can gather, the version from sury.org should be 1.10.9 (full string: "1:1.10.9+submodules+notgz-1+0~20191119.13+debian9~1.gbp296d25").
And/or installing like this should do the trick (if you have everything else already set up - probably worth double checking afterwards):
apt update
apt install php-pear
A bit more googling suggests that once you have all that sorted and if you still get that same issue, then try reinstalling Archive_Tar before retrying again:
pear install Archive_Tar
If you still have issues after that, unfortunately, I'm out of ideas. Although it's probably worth reporting the issue to MS? The issue tracker for the msphpsql
code is here: https://github.com/microsoft/msphpsql/issues