A planeswalker that becomes a creature due to Sarkhan’s first loyalty ability can’t attack unless you’ve controlled it continuously since your turn began.

Once Sarkhan’s first loyalty ability has resolved, each planeswalker you control (including Sarkhan) is no longer a planeswalker for the rest of the turn. They don’t lose any loyalty counters or abilities, and you can still activate their loyalty abilities if you haven’t done so yet this turn. They don’t lose loyalty if they’re dealt damage while they’re not planeswalkers.