Hi @mohamed,
Thanks for providing more detail at here.
Flow will consider calculated column as text, so if you want to compare a calculated column with certain number, you may need to convert it to a string.
If you want to send an approval request when the total vacation days are less than or equal to 21 days. Instead of using Get items, please check the following configuration for a reference.
First, create a list includes columns Date (date and time), Date2 (date and time), and Days (calculated column).

Trigger the flow by When an item is created or modified.
Add a Compose action with the following code to convert Days into a string:
int(triggerBody()?['Days'])
Add a Condition to check if the Compose output is less than or equal to 21.
Under if yes branch, start an approval with details of leave request. Under if no branch, send an email to the requestor.

Please take a try with it and feel free post back if you need more help.
Best regards,
Mabel