I am getting an error saying that 'I can't designate unused RRSP contribution as repayment for LLP' - which I'm not trying to designate anyway - not sure why the error

Payment to HBP has to be designated from this year's RRSP own contribution. 

To break it down, you can have in your profile the following contributions:

1- Own Contributions: made for the current tax year March 2, 2019 - Dec 31, 2019, and Jan 1, 2020 - March 1, 2020

2- Unused Contribution: made in prior tax years, reported in your taxes,  but have not been deducted yet 

3- Spousal Contributions: RRSP you contribute to your spouse's account

HBP designated payment can be made only from the first group "own contribution made in the current tax year". 

Make sure to go back to your RRSP profile and check how you made the designation.