Things you can do when you can't focus

take a walk
do a breathing exercise (e.g. breathing in deeply, holding it for 5 seconds, exhaling, repeating that multiple times)
do a workout
change places - go to the library, a coffee shop,..
if you feel really tired, maybe you need some extra sleep - you can take a 30’ nap with the Calm app which helps you fall asleep & wakes you up after your nap time