Last week only I've experienced the same issue and tried below solutions. You can check if anything works for you.
before came to these solutions - I searched a lot on Google, in Adobe forums, Adobe target documentation but not seen any fruitful solution and even not understand why it's occurring. If someone can tell us the root cause then that really appreciated
1) check if there is any syntax error in code.
2) Try to clear the cache cookies from the browser
rajarajanr71853199 Can you please confirm if the activity is associated with any Workspace, Or are you creating it on the Default Workspace?
If you are using a custom code, you may check for any syntax errors in the custom code.
Hi rajarajanr71853199 ,
Sometimes this error occurs because of audience, so you can try by removing the audience and the see if you are able to publish the activity.
You can also try removing the custom code, if there is any and then check if you are able to publish the activity.
Let us know if this helps in finding the cause of the problem.