It is the order of evaluation. Your commands DiagonalizableMatrixQ[A] and Solve[Det[A - \[Lambda] IdentityMatrix[3]] == 0, \[Lambda]] were given BEFORE you gave a value to A. You should follow the order of the cells.
DiagonalizableMatrixQ[A]
Solve[Det[A - \[Lambda] IdentityMatrix[3]] == 0, \[Lambda]]
A
That solved it, thank you so much, I'm completely new to the Mathematica world, I really appreciate the support. Sorry if it was a dumb question.
