Certificates can have some limitations.
Minimal / maximal sum
You can change minimal and maximal sum of all certificates in the settings.
Other limitations
In addition to the certificate amount, you can specify the certificate lifetime period, the minimum order amount required to activate the certificate, and the maximum discount as a percentage of the order amount.
In the plugin settings, you can change the General limitations that will apply to all newly created certificates. The old certificates will not be changed.
You can also change the limitations in each certificate if necessary.