Excel Solver says "linearity conditions not satisfied" on what appears to be a linear problem, what am I missing?

Hi all,

I have an exam in a few hours with Excel, so I'm quite desperate figuring this out (and I don't think emailing the professor will help in time). Really sorry if it isn't upto the rules. I've tried googling this issue, but I can't find anyone with it.

I'm working on a linear programming problem in Excel Solver to find optimal financing, and I keep hitting this error:

`The linearity conditions required by this LP solver are not satisfied.`

As far as I can tell, my model is linear. Here's what I have:

• No IF-statements anywhere in the model
• The CPM (monthly payment) is calculated using a pre-calculated annuity factor, not the PMT() function
• Decision variables are the financing amounts (green cells)
• Objective is to minimize the future value (blue cell), which is just =-SUM(...) of the above
• Loan taken/remaining are plain input fields — no formulas

When I request the linearity report, Solver flags the objective cell — but I can't figure out why, since it's literally just a negative sum.

Has anyone run into this before? Is there something subtle that can make Solver think a model is non-linear even when it looks linear? Could it be related to how the annuity factor interacts with the decision variables?

Here's some screenshots of the issue: https://imgur.com/a/91YvBHS

Any help appreciated!