See this thread: Can not join any pool after update 2.5.5 - #5 by clydewallace22
This is a known bug in the GUI. The workaround is to use the CLI. The current RC2 has a fix for this.
To use the CLI:
Find your plotNFT wallet id
chia wallet show -w pooling_wallet
Update your pool
chia plotnft join -i (wallet id) -u https://pool.xchpool.org (or whatever pool you want to join) -m (fee in xch, such as 0.000001)