We have recently experienced the following where the last ftp account can't be created.
Server details:
cPanel Version 68.0 (build 13)
Apache Version 2.4.29
PHP Version 5.4.45
Architecture x86_64
Operating System linux
Perl Version 5.10.1
Server is running Centos 6.9.
See attachment:

Some experimenting suggests that it may occur when you hit as we saw this when we hit 12/15.Error: You have already used your maximum allotment of (10) of FTP (File Transfer Protocol) accounts.
Server details:
cPanel Version 68.0 (build 13)
Apache Version 2.4.29
PHP Version 5.4.45
Architecture x86_64
Operating System linux
Perl Version 5.10.1
Server is running Centos 6.9.
See attachment:
