Hey is sololearn certificate valid (can it be posted on linkedin just like Coursera and Udemy certificates)?

Answer 1

Sure it is "valid" in the sense that it is issued by Sololearn, a real company, based on your real achievement of completing the course.
But the practical worth is probably much less than completing a full blown course on Udemy or Coursera or any learning platform, depending on the actual course and what happens there at the end, like a proctored exam, or timed test, or code assignment validated by the teacher, or none of these things just watching some videos...
See my answer to this other topic too.
https://www.sololearn.com/Discuss/3181424/?ref=app