Once proposals are accepted they can be billed follow these steps

Open the Proposal
Select Accept Proposal
Select Save
Open a Sales Invoice
Select Customer
Select Proposal
Select one of the 3 options: The remaining percent to bill of the Proposal (including freight), A percentage of the Proposal. % to Bill (enter the percent here), or Manually enter different percentages or amounts […]

Resolution for 'Accept Proposal issue: 'How to bill a proposal' available: Yes.

The Accept Proposal issue or error code is a known related to Sage Accounting products. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Accept Proposal issue yourself by looking for a resolution described in the article. If it is a complex Accept Proposal issue or you are unable to solve the issue, you may contact Sage Repair by clicking here or by using other Accept Proposal support options.

For more information about resolving issues related to Accept Proposal in Sage Accounting or to get support for the issue 'How to bill a proposal', click the link below: