The precise message is "calibration error - "solver not found. this workbook will not work."
The Solver is part of Microsoft Excel, and is not installed initially. This error message pops up for users who have never used the Excel's solver on their machine. To use this "Calibration" functionality in NumXL, the Solver add-in must be installed.
To fix this issue, go to the Add-in manager, and click/check the solver entry. This will trigger the installation of the solver, and it may prompt you for Office installation media.