A JKS cannot exist without a key and a certificate in it.
You are importing a certificate to an existing key.
You need to convert your key and cert to a PFX and then import that.
A lot of theory here: Tutorial - Java KeyStores (JKS) With Let's Encrypt
How to do the PFX: Certbot and Tomcat - Certificates Obtained but Not Converted to JKS and Configured with Tomcat (FIXED!)
Andrei