Someone told me that I can't get married by the koisel.
Is this true?
If yes what is the reason, (religious (then sources please) or practical...)?
Like others, I couldn't think of a single halachic reason to forbid weddings. I know I've seen dozens of Bar Mitzvahs over the years there, but never a wedding. (On Monday and Thursday around mid morning it seems like there's a bar mitzvah every 20 minutes). I did some digging and eventually found the rabinate's rules for the kotel, and it is true that they forbid both weddings and britot at the kotel. (They also have lots of rules around sound amplification). It appears to be a rule based on purely practical reasons.
Somewhat tangentially it is common (common? Okay maybe not common but certainly not unheard of) for brides to go to the kotel before their wedding to pray, often times in their wedding dress (which usually elicits lots of "mazal tov"s from onlookers). The Temple Institute even cites a tradition for a bride to ascend to the temple mount after her first mikvah immersion.