Problems with IK for SFM Imported model

Hello, I have recently been trying to work with a Anthro Shark model from Warfaremachine on deviantart https://www.deviantart.com/warfaremachine.
Anyway, with everything I have tried the model doesn’t want to work with IK. Could some one help me as I am decently new to IK and I don’t know what I am doing wrong or if it is a problem with the fact it is imported from SFM
The link for the model is:
Link to model (mega.nz)
(The model is slightly NSFW but I have tried to remove the Vagina and have edited the textures a little bit)
Thanks in advance