QB Issue Resolution:

the customers are paying you, that is your income
you pay the trainer, that is your expense, it is not cogs, there are no goods being sold

you should track the trainer as a vendor eligible for a 1099 just in case he gets paid $600 a year or more in cash or check

Resolution for Issue 'Commission for Sale of Service vs. Purchasing Service from Provider' available: Yes (Solved).
Source: Intuit Community forum.

Commission for Sale of Service vs. Purchasing Service from Provider: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Income
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Error Recovery for Income [Canada]
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