On Jan 1, 2020 all self signed certificates that were generated on IOS/IOS-XE platforms got expired. After that time your device will not be able to generate self signed certificates.

There are some work arounds :

• Obtain a valid certificate from a 3rd part Certificate Authority
• Use the IOS CA Server to generate a new certificate
• Use OpenSSL to generate a new self-signed certificate
• Update your system, if possible, to a fixed SSC IOS version

Systems affected :

• All IOS 12.x
• all IOS 15.x prior to 15.6(3)M7, 15.7(3)M5, 15.8(3)M3, 15.9(3)M
• all IOS-XE prior to 16.9.1

You can find more information here.