You can actually bypass the slenderness check if that is not important. Not sure which design code you are using but for almost all of codes , you can use the MAIN parameter to bypass the slenderness check for compressions members. For members in tension there is a similar parameter called TMAIN in some codes. You may have to set the MAIN appropriately to bypass the slenderness check, for example 1 ( for AISC 360 ) or -1 ( for IS 800 2007 LSD ) etc. The value required to be set will be explained in the appropriate dialog box for the parameter and also in the documentation for the design parameters related to that code. Also to have these parameters affect the design, ensure that these parameters are specified ahead of the design commands like CHECK CODE.
If you still get slenderness failure, upload your model and mention which specific member number you are checking and we will take a look.
If you still get slenderness failure, upload your model and mention which specific member number you are checking and we will take a look.