What to do when bladder is full and penis is erected and you are unable to urinate normal? and the reason?

Sounds like: mechanical blockage caused by erection that is preventing bladder emptying, which is not unusual and probably normal, especially if you can urinate normally after erection goes away. You obviously have a urologist who is prescribing your meds and performed surgery, so you should ask him/her about this immediate problem.

Normal physiology: Bladder neck contracts when U have erection. Thus semen passes down urethra, rather than backwards, when U ejaculate. Thus urination difficult, & in some impossible, during erect state