My 2020 State tax return has a large amount entered as a PFL Insurance Benefit in Schedule CA, "Wage, IRA and Pension Adjustments." Now I owe tax. Why did this happen?

Since you mentioned Schedule CA, I'm assuming the state return is for California. Please let me know if you're filing for a different state.

Whether Paid Family Leave (PFL) benefits are taxable in California depends on who paid the benefits. If the benefits were paid directly by your employer, the benefits are taxable as wages. If the benefits were paid by your employer's insurance company, the benefits are not taxable in California. (The benefits are taxable on the Federal return either way.)

Verify that your PFL payments came from an insurance company (check the W-2 or your payment statements). If the benefits did come from the insurance company, adjust your Schedule CA to remove the PFL benefits from your California taxable income.