I need some help in transaction fee in Bitcoin Core.
In Send section there is a message after Transaction fee saying "Warning: Fee estimation is currently not possible." Why is it there? Blocks are up to date.
The Bitcoin Core software needs to have been running for a little while before it is able to provide fee estimates.
This is because the fee estimation mechanisms observe unconfirmed transactions on the network and the time it takes for them to confirm. With no (or little) observation, it's not possible to provide an estimate.
For more details about how fee estimation work in Bitcoin Core see this answer.