I just recently bought a 99 CBR600F4
2nd gear won’t catch.. I can shift into it I feel the bike shift into it then I give gas it goes into N..the chain is a lil loose I’m goin to adjust it and change oil .but I think there is something more wrong

 

Have a look into whatever you catch your oil in. Look for debris.
Best of luck mate.

 

Worn dog in the tranny maybe?
I couldn't say for sure But may be something to look into. Worn dogs on the gear would cause it to keep slipping back into N from 2nd.
Could be a bent shift fork , but you would likely have troubles with more than 2nd if that were the case I would think.
Definitely would check anything else suggested first , since its a little more involved than an Oil change.
Hope you find an easy fix.