I would check the version of snmp_ta you are using and the version of Splunk you are trying to get working on. I suspect that you need specific versions of each to work together. There are also some other answers which suggest some possible issues in Splunk v8 - https://answers.splunk.com/answers/144720/snmp-input-type-not-displaying-in-splunk-web.html
Please also note that the SNMP modular input (I presume you are using this - https://splunkbase.splunk.com/app/1537) now requires an activation key and so may not work otherwise.
Personally I could not get the SNMP modular input to work reliably - it would often fail on complex data and getting the MIB files converted to work is a big pain. Depending on the type of device you are collecting from, MIB's are essential to get any sort of readable data.
My preferred approach is to use http://www.net-snmp.org/. Just create a standard batch/shell script using snmpwalk to collect the data. Not only can you easily test outside of Splunk, but it allows you to collect large number of different targets by just expanding your script. You then create a scripted input in Splunk to run the script on your required frequency. If can get a script working outside of Splunk, it is not too difficult to then get it working inside Splunk.
if you want to stick with the modular input, here is some info - https://www.splunk.com/en_us/blog/tips-and-tricks/making-snmp-simpler.html
These two articles are also very helpful, especially around transforming SNMP results when they are received which is useful whether you are using the modular input or another script.
https://www.splunk.com/en_us/blog/tips-and-tricks/adventures-with-snmp-and-cisco-nexus-pt1.html
https://www.splunk.com/en_us/blog/tips-and-tricks/adventures-with-snmp-and-cisco-nexus-pt2.html
