Activate Windows 11 Pro

I have tried to activate Windows 11 Pro on my LattePanda Sigma.

But there is no license to look for. Because a license is apparently not included !

What's my and everyone else's best option ?

I don't want to buy a normal Windows 11 Pro license for $150 - $250.

Can I buy a cheap OEM license for $15 ?

Or is it a cheap Retail license I should choose for $20 ?