Can you still subben with the free Twitch Prime?

I've had the free Twitch Prime for 5 days and can't subben anyone.

I have already tried to disconnect and reconnect my account with Amazon, Twitch Prime Gaming several times

Do you mean prime what you get for free as a student or trainee? You can't subben with that, only if you pay Prime yourself.

No free Prime last year and for some even in April

And under "My Subscriptions" is "Free channel subscription with Prime"