Problem with useradd on Openwrt-19.07

Hi everyone,

I have 2 problems with useradd:

1:When I added this app in my image I could not use passwd and I have below error:

passwd: failed to crypt password with salt '$6$WvRuLtmif./E3ps': Function not implemented
The password for test is unchanged.

But when I did not use adduser I can change my password.

2:When I try create username with uppercase character as follows :
useradd -m -s /bin/bash Admin
Then I have this massage :

useradd: invalid user name 'Admin'