I'd like to downgrade from Essentials to Simple Start but the error "Unable to downgrade" is shown. What should I do to downgrade?

The error you’re seeing occurs because the Multi-currency feature is enabled in your Essentials plan, @usastaff.

Currently, the Simple Start plan doesn’t support the Multi-currency feature, and the option to turn off Multi-currency once it has been activated is unavailable.

As an alternative, I recommend creating a new QBO Simple Start plan under a new subscription. Here’s how:

1. Go to Plans & Pricing and click the Choose Plan option for Simple Start.
2. Choose a Payroll (optional), and click Add to plan.
3. Review your plan and create or sign in to your Intuit Account.
4. Enter an Email, Phone number, and Password.
5. Click One more step, and follow the last step.

You can check this site to view the features of each plan: Products & Features.

Once your new account is set up, you can manually export and migrate your data from your current account to the new one.

However, if Multi-currency is still an essential feature for your business, I suggest continuing with the Essentials plan.

Please feel free to comment below if you have further clarifications.

@usastaff 

Try converting your data to QB Desktop as the first step and converting it back to a new QBO Simple Start account.