What can't the VB compiler find dependencies?

I am using VS2017. I have an app written in VB that has been running fine for months. This morning it would not run. I tried to recompile and got the following warnings.

Severity Code Description Project File Line Suppression State Warning Could not determine the dependencies of the COM reference "Microsoft.Office.Interop.Excel". Element not found. (Exception from HRESULT: 0x8002802B (TYPE_E_ELEMENTNOTFOUND)) PG Checker

Severity Code Description Project File Line Suppression State Warning Could not determine the dependencies of the COM reference "Microsoft.Office.Interop.Outlook". Element not found. (Exception from HRESULT: 0x8002802B (TYPE_E_ELEMENTNOTFOUND)) PG Checker

Tried deleting project and rebuilding but nothing. The app uses a list contained in an Excel workbook to look for emails with specific Subjects or files that have been updated in the current month. The data is collected for monthly reports. It has worked fine until this morning.

TIA Frank