Forms show an error about being read-only in Dynamics 365 Field Service
This article helps resolve an error message that occurs in Microsoft Dynamics 365 Field Service forms.
The error message occurs in the Dynamics 365 Field Service forms:
This form can only be used on service-maintenance based records. Record is read only.
The Dynamics 365 Field Service makes the out-of-the-box Field Service opportunity, lead, quote, and invoice forms read-only once Field Service detects the record isn't a service-maintenance based record. Field Service forms or the forms copied from Field Service forms don't support opportunity, lead, quote, and invoice records other than Field Service opportunity, lead, quote, and invoice.
To use an opportunity form for non-Field Service-related opportunities, use the out-of-the-box opportunity (Opportunity: Opportunity) form or create a copy of the Field Service form for further customization. The same applies for the Field Service forms for lead, quote, and invoice.