On videos marked "for kids" (which can be common videos too, but marked by the creator as such, just to turn off comments), save to playlist button is (uselessly) disabled, to make it compliant with the COPPA law.
❗ Solution
Find the litho object related to this button and unlock it.
❓ Motivation
Because I never seen as such stupid (and annoying) limitation.
⚠ Additional context
Despite this limitation, these videos can still be added to a playlist with 3-dots menu outside the player.
oSumAtrIX
commented
2 years ago 
🐞 Issue
On videos marked "for kids" (which can be common videos too, but marked by the creator as such, just to turn off comments), save to playlist button is (uselessly) disabled, to make it compliant with the COPPA law.
❗ Solution
Find the litho object related to this button and unlock it.
❓ Motivation
Because I never seen as such stupid (and annoying) limitation.
⚠ Additional context
Despite this limitation, these videos can still be added to a playlist with 3-dots menu outside the player.