bubblelegend was released in R2020b, so it should work in R2021b. My guess is you have something wrong with your MATLAB path. Try this:
restoredefaultpath
rehash toolboxcache
If bubblelegend works after that, then there was something wrong with your path. Most likely you have a copy of pathdef.m in your user directory, which is overloading the official version. You can confirm that with this command:
which pathdef
If the answer is in your user directory (something like C:\Users\<username>\Documents\MATLAB) then that is the problem. The file in that directory will be used by all versions of MATLAB, but pathdef.m needs to be different for each version of MATLAB. You can try deleting that file, but first make sure you've captured any customizations you've made to the file.
If that doesn't help, or if you need further assistance, I suggest contacting technical support.
