The User Skills Reporting API uses a different 'offset' than most of our other APIs.
You'll want to start by pulling all records, this can be done by using the since parameter with a date in the past. This will look something like this:
https://company.leading2lean.com/api/1.0/reporting/skillsdata/usertraining/?auth=&site=3&since=2021-01-01%2000:00:00
Then you would use the ID from the last record returned along with the since parameter until you return all results.
https://company.leading2lean.com/api/1.0/reporting/skillsdata/usertraining/?auth=&site=3&since=2021-01-01%2000:00:00&lastid=5498
Note: If you are using an integration service such as StreamSets, you will need to make sure the offset is not automatically occurring. This will create an endless loop instead of paginating through to the end.