Welcome!

To force users to select a task:

• activate the developer mode and go to settings->technical-> automation rules (in Odoo enterprise it can also be done opening studio from the timesheet app)
• create the following automation:

code:
if record.is_timesheet and not record.task_id:
    raise UserError("Please select a task!")

or force users to enter a sales order item or prevent the use of sales order items that should not be used.

• activate the developer mode and go to settings->technical-> automation rules (in Odoo enterprise it can also be done opening studio from the timesheet app)
• create the following automation

code:
if not record.so_line:
    raise UserError("Please select a sales order item for this timesheet entry.")
elif record.so_line and 'do not use' in (record.so_line.name.lower() or ''):
    raise UserError("The selected sales order item contains 'do not use' and should not be used.")

both use cases can also be handled in the same automation.