Rockrider btwin 6.2 swingarm pivot
First thing to do is check all the pivot bolts are tight. If that doesn't fix it, figure out where the play is. Is it in the swingarm pivot, or the shock bushings? You can figure it out by holding onto each joint in turn and moving the swingarm. You shoukld be able to feel which part is loose. If it's been like it for a while, it might be all three points.

Once you find the source of the problem, I guess you'll need to go to your local bike shop (or the place you got the bike - Decathlon?) and see if they're able to source the spare parts. Shock bushings should be easy enough to get, but I don't know about the pivot hardware. You'd need to open it up and have a look what's in there!