From Catalina onward, all 32-bit apps will stop working. This includes legacy Apple Java 6.
inTennis seems to be fine as long as a 64-bit Java is installed.
For some reason Catalina is not allowing to install Oracle Java 8 JRE (or JDE). In this case install OpenJDK instead from here
If Catalina complains it cannot check inTennis (despite having correctly signed by Apple certificate), then to run it
- Right click
- Hold CTRL key and click Open