Why might the key not be suitable for PuTTY?

Question

PuTTY

I created a pair of keys, put the public one in ~/.ssh/authorized_keys, created .ppk from the private one. I try to specify it in PuTTY, I get putty server refused our key.
The private key works, I can use it to ssh into phpstorm.
The rights to folders and files on the server are correct.
What can be wrong?
In log/secure such lines

pam_succeed_if(sshd:auth): requirement "uid >= 1000" not met by user "root"
Oct 5 09:55:19 CentOS-71-64-minimal sshd[23691]: ​​debug1: PAM: password authentication failed for root: Authentication failure