If I used a Ready action to attack, can I still use my Reaction to attack on the same instance?

Question

I use a Fighter with the Polearm Master and Sentinel feats that has a reach of 10ft.

On my turn:

• I used Ready as an action in order to attack any monster that came within my reach.

• End my turn.

Enemy turn:

• Enemy moved inside my reach which activates my Ready action. (1st attack).

• Used my Reaction to attack since he entered my 10ft reach thanks to Polearm Master (2nd attack).

• The monster's speed is now 0 and it can't reach me because of Sentinel.

Is this legal? Or can't a ready action overlap with reaction?

Answer 1

This doesn't work because to take your Readied Attack action when the enemy moves into your reach, you use up your Reaction.

PHB, pg.193:

Ready

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn so that you can act later in the round using your reaction. [...] When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can only take one reaction per round.

Since the attack from Polearm Master is an opportunity attack, which also uses your Reaction, you can either choose to use your Readied action to attack or make the opportunity attack with Polearm Master but you can't do both because once you've taken a Reaction you can't take another one until the start of your next turn, as per the reaction rules on page 190 of the PHB:

When you take a reaction, you can't take another one until the start of your next turn.