You can never delete a priority in an adopted programme, but you can modify the programme as follows:
If you no longer want to use priority X:
-> In T11 add the amounts of priority X to the amounts of priority Y, and set all amounts of priority X to 0,
-> In T2 for Priority X set the milestone and target values to 0,
-> In T3 for Priority X set the baseline and target values to 0,
-> In T4, T5, T6, T8 for Priority X remove the rows.
After that, you should be able to validate without any errors and submit.