I have this same problem after upgrading from Splunk Enterprise 7.2.1 to 7.3.4 with the previously working 5.0.0 add-on.
I have even gone so far as to stop Splunk, delete the add-on, start Splunk, stop it again, untar the 5.0.0 (and 5.0.1!) add-on, and start Splunk. Both versions display the spinning wheel.
I've copied the Splunk_TA_snow/default/inputs.conf to Splunk_TA_snow/local/inputs.conf with no change in behaviour.
Relevant logs:
$SPLUNK_HOME/var/log/splunk/splunk_ta_snow_main.log
2020-02-19 22:13:51,432 INFO pid=71324 tid=MainThread file=snow.py:stream_events:288 | No configured inputs found. To collect data from ServiceNow, configure new input(s) or update existing input(s) either from Inputs page of the Add-on or manually from inputs.conf.
$SPLUNK_HOME/var/log/splunk/splunkd.log
02-19-2020 22:20:48.589 +0000 INFO ModularInputs - No stanzas found for scheme "snow" in inputs.conf at script (re)start.
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