SSRS 2008: error message saying my parameter doesn't exist but it clearly does?

Try changing the order of the parameters using the up/down arrows.

They should appear in the order of dependence.

I found in my situation that it was actually the parameters name casing was slightly changed and it was throwing this same error.

I had to change the name to the correct casing, and then search in the code behind to anything that refers to that parameter and correct its casing. Deployed fine after that.

Re-ordering the report parameters based on their dependency fixed the issue for me. I had the report parameter at the bottom of the list but the one above it was dependent on this bottom one. So when you are creating / re-creating the report parameter make sure the order is right.