Please follow the below instructions to activate Burp:

Run : java -jar burploader-old.jar
Copy the license key.
Run : java --illegal-access=permit -Dfile.encoding=utf-8 -javaagent:BurpSuiteLoader_v2020.9.jar -noverify -jar burpsuite_pro_v2020.9.1.jar
Use manual activation proces
Have fun