This is another example of the VueJS's project causing us all problems. They prematurely switched the default version from v2 to v3. Vue v3 has significant changes which means that it takes a long time for other projects such as bootstrap-vue to be able to catch up.
To get round the problem, you will need to simply click the add button in the libraries tab and put in "vue@2" then click install. The Vue v3 install that you currently have will be replaced by Vue v2 and everything should work.
I missed the part of the walkthrough that needed the instruction changing. It will be corrected in the uibuilder v6.2 release.