I suspect you are trying to post to the salary expense account on your trial balance or income statement.

These are total to accounts and cannot be posted to directly as you have notices.
You can only post budget amounts to posting accounts.

When we ran into this problem there are 3 ways we found to overcome it.

1 post your budget to all the employees account on the payroll record. If you only have a few employees this is not that bad.

2 create a payroll budget report and create budget accounts that total to your salary expense account on income statement or trial balance.

3 create a fake employee named budget on your payroll record report, create all salary accounts for this employee. For example if you have salesmen that have commissions but management that have bonuses you will have to create both accounts for the fake employee. Post the budget amounts to this employee.