Users cannot delete threads.
Discourse uses some kind of trust level system for many features: https://blog.discourse.org/2018/06/understanding-discourse-trust-levels/
It looks like deleting topics is only available to moderators by default: https://meta.discourse.org/t/trust-level-permissions-table-inc-moderator-roles/224824
Is there a specific topic you would like to have deleted? Feel welcome to message me in private.