Why can't i get more lindens if i have a set up payment plan?

[Sashia Foxtrot]

can't get more lindens,i have more than enough in my credit card...

[valerie Inshan]

Adding to Roseysun's answer about trading limits, if you mean tou cannot buy L$ NOW, it is possibly because of this:

 

Unscheduled Maintenance

[Posted 8:26am PDT, 13 July 2012] We are currently undergoing unscheduled maintenance to our systems. This may prevent some residents from logging in (or cause them to be logged out), or may affect other inworld services. In the meantime, please refrain from rezzing large builds or making L$ transactions. Please watch this blog for further updates.

[Lindal Kidd]

And to add yet another morsel...if you have just now set up your payment method, your trading limits are held low for a couple of days, but are then automatically increased.  This is a measure taken to combat the use of stolen credit cards to purchase $L.