In the Sales and marketing parameters in D365 F&SCM, set the Opportunities parameters to 'Never'.
If the integration between D365 F&SCM and Rental Management for D365 Sales is active, opportunities are managed in Rental Management for D365 Sales. So, it is not desirable that opportunities are created in D365 F&SCM.
If not set to 'Never', on synchronization of (project) rental quotations from Rental Management for D365 Sales to D365 F&SCM, an error message is shown.
Steps
Go to Sales and marketing > Setup > Sales and marketing parameters.
On the General tab, set all Opportunities parameters to 'Never'.