Currently, when you do not have tariffs configured, the reset-service is not activated.
See: Source code
In this code, registration of the reset-service is only done when tariffs are configured.
As a result, when you want to reset a UM-entity, you have to use the calibrate service.
For example:
service: utility_meter.calibrate
target:
entity_id: sensor.boiler_zolder_bijlading
data:
value: '0'
Unfortunately, this offcourse does not set the attributes last_reset and last_period like the reset function would do.
This feature was present in early 2019, but got removed in this PR.
However, it is unclear if this was by accident or not.
The related issue also states this.