This can be due to an inactive block being included in the Schedule Set. This block would either need to be marked as Active, or replaced on the Schedule Set with the correct block.

To mark a block as Active:
  1. Go to Core > School > Blocks.
  2. Select the School Level.
  3. Under Status for the block, click on Inactive.
  4. Select Active from the drop-down.
To adjust the blocks in use on a Schedule Set:
  1. Go to onRecord > Settings > Scheduling Setup.
  2. Select Schedule Sets.
  3. Select the School Level.
  4. Click the Pencil icon next to the schedule set to modify.
  5. Select the Rotation tab.
  6. Locate any meeting times with the inactive block selected and change these to the correct block.
  7. Click Save & Close.
  8. Repeat steps 4-7 for any additional Schedule Sets where this block is in use.