Nice to have you in the Community. I'm here to help share some information about the sales tax setup in US QuickBooks Online version.
In QuickBooks Online, if you're using the automated sales tax, you can always choose to move back to manual setup. However, since you're using the QBO US Version, we cannot use Mexican VAT because the taxes are only compliant to the US. If you wish to set up for Mexican VAT, you'll need to use the QBO international version.