Hello there, uservalentina.paolet.

Let me share some information with you about the message you're getting.

You can only accept clients under the same QuickBooks Online (QBO) region. However, you can let them invite you as the Company Administrator for you to access their company file. 

I'll show you how:

1. Go to the Gear icon.
2. Select Manage users.
3. Select the Accounting firms tab.
4. Select Invite.
5. Enter your accountant's info and select Save.

For more details about managing an accountant user in QBO, please check out this article: Invite an accountant to review your accounting in QuickBooks Online.

I'll be around if you have any other QuickBooks related concerns. Have a great weekend @uservalentina.paolet.