Honestly, I can't tell you the chance of securing a visa with your conditions since there are a whole lot of other factors involved. Securing an admission letter, even if it's a conditional admission letter, tells the Embassy the university believes you are fit to pursue their degree program.
Your IELTS score is a little low. However, your university has offered you the chance to retake it in Germany. The question the Embassy will ask themselves is what is the guarantee you will get the required score if granted a visa.
So you should weigh the options and decide if it's worth it to apply. I can't decide for you. In case, you decide to apply, work extensively on your motivation letter. Explain in your motivation letter why you had a low score in IELTS and convince them you will get a higher IELTS score and pass your aptitude test if given a visa to study in Germany.