TOL and CTOL are built-in values (like ORIGIN.) They control the actions of many of the built-in solvers by setting the points at which the solution is considered "converged." Adjusting their values to greater or lesser values may allow Mathcad to reach a "solution."
Are you sure that the data is correct?
If you check columns 2 and 6 these only have the 1 entry so you only have 1 equation that uses N2 & N6,
The best that you can achieve manually is a*N2 + b*N6 = c.
Since you then have 6 equations with 5 unknowns, it is highly likely that Mathcad will not find a single solution that fits perfectly & will stop with an error message.
As already written here Solved: Re: tol and ctol - PTC Community
your system has no (unique) solution.
You can easily check by calculating the determinant of your matrix - its zero!
When you use a solve block, you are using a numeric method and so Mathcad does not know why it can't find a solution. It may be becuase the problem has no solution (as in your example) or because of numeric problems. The latter is the reason why Prime suggests to use different guess values and/or play around with the values of the built in variables TOL and CTOL. In your case obviously this would not help.