You need to surround both sides of the SHEET ID with ".
In your example, you only have " at the start of the SHEET ID. The double-quotes need to be on the end of both SHEET ID references too.
Also, sometimes copying single and double quotes can be a problem, if you browser turns them into starting and ending quotes.
What you copied uses starting and ending quote (not sure if that’s just in the Community or also in your spreadsheet too?)
=FILTER(IMPORTRANGE(“SHEET ID,”Transactions!A:L”), INDEX(IMPORTRANGE(“SHEET ID,”Transactions!A:L”),0,4)=”Groceries”)
They should look like this: (regular double-quotes and quotes added after SHEET ID):
=FILTER(IMPORTRANGE("SHEET ID","Transactions!A:L"), INDEX(IMPORTRANGE("SHEET ID","Transactions!A:L"),0,4)="Groceries")