Hello Emily,
That error comes up generally when there is a spelling or punctuation (including spaces) mistake and thus the code cannot find the sheet it needs to interact with.
Check that all spelling etc. is exactly the same in the code, the sheet tab name and in your data set (the criteria). The code is case sensitive.
"Ideally i would like - if the drop down option of successful is clicked, then it automatically transfers the row to the successful projects tab"
The code does exactly that.
If you don't need any rows deleted, then remove this line from the code:-
cell.EntireRow.ClearContents
Cheerio,
vcoolio.