There are a couple of reasons you may not be able to select VAT on your expense, see below for more information.
The VAT fields are greyed out
If the VAT Rate and Amount fields are greyed out on your expense, the system will not allow you to assign a VAT amount as it sees the expense you're creating as a foreign expense.
If you have any queries on this, contact your company's Finance team.
No VAT rate available to select
If no VAT rate is available for you to select from the drop-down on your expense, contact your company's Finance team as they'll be able to make the relevant rate(s) available to you.