[Python] User.mutual_guilds returning an empty list

I decided to code another bot, but this time I coded it with an on_message support function. It worked perfectly fine until I decided to let the user choose which Server the problem relates to. So, i passed ctx (basically the message), and used self.ctx.author.mutual_guilds. And now, I ran into problems, because mutual_guilds returned an empty string. Can anybody please explain me why this happens?

class ChooseTicketServer(Select):

def __init__(self, ctx, id):
self.ctx = ctx
self.id = id
self.ServerOptions = [SelectOption(label="Test", value="Test")]
self.servername = None

print(self.ctx.author.mutual_guilds)

for guild in self.ctx.author.mutual_guilds:
self.ServerOptions.append(SelectOption(label = guild.name, value=f"Server-ID: {guild.id}"))

super().__init__(placeholder="Choose the Server your Problem relates to...", min_values=1, max_values=1, options=self.ServerOptions)


class SupportViewCreate(View):

def __init__(self, *, timeout: float = 120.0, ctx, id):
super().__init__(timeout=timeout)
self.ctx = ctx
self.id = id
self.add_item(ChooseTicketServer(ctx=self.ctx, id=self.id))


@bot.event
async def on_message(ctx):
if ctx.author == bot.user:
return
if not ctx.guild:

id = random.randint(0, 99999999999999999999)

while id in bot.activeSupportIDs:
id = random.randint(0, 99999999999999999999)

bot.activeSupportIDs.append(id)

embed = Embed(title="Support Ticket", description="Please choose the Server your Problem relates to.", color=discord.Colour.green())
embed.set_author(name=f"{ctx.author.display_name}#{ctx.author.discriminator}", icon_url=ctx.author.avatar)
embed.add_field(name="Ticket-ID: ", value=f"#{id}")
embed.set_footer(text=f"This is an automatically sent message from {bot.user.name}.")

View = SupportViewCreate(ctx=ctx, id=id)
await ctx.channel.send(embed=embed, view=View)

else:
await bot.process_commands(ctx)

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