How can I partition my disk?
Okay I solved the problem. First I created a backup, because everything got erased!


I restarted my MacBook in Recovery mode (Command + R)
Went to the Disk Utility
Clicked on my SSD that was in APFS
Clicked on Erase, and chose Mac OS Extended (Journaled) with GUID Partition Map
After it was done, went back to the previous menu in Recovery mode and clicked to Reinstall Mac OS
I chose the new disk to install macOS (it took me around 30-60 minutes)


After installing macOS you can set your previous Time Machine backups. Finally I'm done, and now it enables me to partition my disk.

The problem was the APFS didn't enabled me to partition, only HFS+.

But be careful: since APFS is now the standard filesystem on Apple devices and if it has some important advantages which is not supported by HFS+ you will be messed up.

Take a look at APFS advantages before you choose.

Although you can reset your disk back to APFS anytime you want, but you might miss the ability of partitioning.

I think if you want to keep your drive as APFS you can purchase an external hard drive (it doesn't cost so much, but indeed needs extra care if you are working with a laptop).

Edit: After I erased my SSD with HFS+, I tried to erased it with APFS once again. (the whole SSD) And this time, it enabled me to partition in APFS. :)