i have some lumens and i can't transfer them - Insufficient balance. To maintain your minimum balance, the most you can currently send is 0.999901.
how can i send more? everyone knows?
I'm guessing you have around 21 XLM at that address. This is because the stellar network requires every account to always have at least 20 XLM in it if you want to send funds. There is a way to recover these 20 if you want by merging addresses, but I've never tried it.