multiple selection widget I am using Drupal 8 and I cannot find where the multiple selection checkbox for my taxomomy field is.


And the result is radio buttons.
You have selected field cardinality to one . So its not reflecting checkbox.
Please follow the below steps you can achieve what you asked 


Go to Structure > Content Type.
Select the {{Custom Content Type}} . Click on Manage Fields.
Edit the taxonomy Entity Reference field.Click on Field Settings Beside Edit Tab
Change Allowed number of values from Limited to Unlimited