Solutionizing is when you create a solution, but the problem your solution is trying to solve has not been defined or, even worse, might not even exist. This happens when you make presumptions about what a problem may (or may not) be, and then you create a solution without considering the actual problem—or even ignoring that no actual problem exists.
Although my business analyst colleagues and I used it a negative sense, to mean roughly:
'trying to (or being asked/forced to) design the solution when your job is only to research the problem and articulate the requirements'.
Edit: similarly, https://www.iltanet.org/blogs/michael-ertel1/2021/07/26/whats-your-problem-dont-solutionize