What to do when the 20kg capacity wallmate doesn't go in the wall?

depends on what the metal part is

eg is it a lightweight metal wall frame stud?

If so drill thru it and ignore as a problem.

It may be a solid steel strut for example. If so could be need a major drill through.

It may be a water pipe, electrical conduit. keep away.

Can you locate the steel's path and maybe drill above or below the steel item?

Into clear through material.

Sometimes its an advantage to use a steel backer as the hanging point as it is stronger.

If you don't have strong steel drilling bit and power drill i'd try to work around the steel .Will it matter if you go up or down eg 1 inch?

If you don't have a cheap studfinder, borrow one or..

Be prepared to spot drill above or below with a small eg 1/8 " drill . Till you find a clear hole through .Fill with acrylic filler maybe afterwards.

Holes in walls are the DIYers biggest nemesis.

I have about 20 different drill bits and several types of power drills to get around problems like yours.

My advice to any home handyman..

Always allow twice as long as you think it will take.

Always assume something will go wrong.

Make sure you have the part BEFORE you start.

Mark your holes / lines etc with white masking tape and a sharp dark pencil. Easy to change or remove then.

MAKE SURE YOUR WIFE/ BOYFRIEND IS HAPPY ABOUT YOUR LATEST DIY BRAINWAVE or you will regret it.

People don't always want to pay a tradesman to do a job.

Find his apprentice . He'll do it cheaper . Usually for cash.

Be nice to your tradesmen , You will benefit.

He'll have the drill etc to do it.

I do lots of jobs like yours. Bad customers who tell me how to do a job only get 1 visit.

I can get plenty of work from sensible customers.

Follow the above...

Then a problem arising is just something to be solved.

Not a catastrophe.

Hope this may help