You need an active payroll subscription and then it should work.  I think you need Enhanced payroll:

https://books.google.com/books?id=WbQ55tZH3JsC&pg=PA168&lpg=PA168&dq=quickbooks+enter+net+calculate+…

Resolution for 'Calculate gross issue: 'How do I get the grayed out box “enter net/Calculate gross” to work for a paycheck? This has nothing to do with sales tax.' available: Yes.

The Calculate gross 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 Calculate gross issue yourself by looking for a resolution described in the article. If it is a complex Calculate gross issue or you are unable to solve the issue, you may contact us by clicking here or by using other Calculate gross support options.

For more information about resolving issues related to Calculate gross in Quickbooks/QBO or to get support for the issue 'How do I get the grayed out box “enter net/Calculate gross” to work for a paycheck? This has nothing to do with sales tax.', click the link below: