Word Reference includes both 'high skilled' and 'highly skilled' in its list of (...)
Forum discussions with the word(s) "skilled" in the title:
- high-skilled workers
- highly skilled workers
...
Collins Online Dictionary and Cambridge Dictionary list 'highly skilled' but not 'high-skilled'. Merriam-Webster includes only an example of the former.
But Thesaurus.com includes examples containing both 'highly-skilled' and 'low-skilled':
Highly-skilled professionals across sectors have a leg up when employers cherry-pick roles to reward. [Quartz]
However, the enormous benefits that low-skilled immigrants provide more than make up for that relatively small cost. [Daily Beast]
(but of course, 'lowly skilled' would be farcical, with or without a hyphen).
A Wikipedia article contains skilled, unskilled, semi-skilled, non-skilled and highly-skilled, as well as "Obama Immigration Order to Impact Millions, Includes Provisions for High-Skilled Workers".
Although many credible examples for the collocation/compound adjective 'high skilled' can be found on the internet, Google ngram results would indicate a 5 : 1 ratio in favour of 'highly skilled' at the moment.
So the best policy is to choose the favoured form, and not be upset if someone chooses the valid alternative.