The stitch length dial/knob is stuck
Stitch length button / dial is stuck.
In these cases there is often an axis or exenter of the stitch lenght button stuck inside.
This is often due to old oil residues and dirt.
You can usually access this from the back of the machine.
Try to find out exactly where it is stuck by gently moving the knob.
Remove all dust from these parts well, and then clean with a degreaser, eg contact spray
Then spray the parts well with penetrating oil, eg WD 40
Let this work for a while and then carefully try to separate these parts by moving repeatedly.
This often takes some time and effort.
When the parts are loose, clean everything well and lubricate the machine with sewing machine oil.
During the following days, check regularly that it does not get stuck again, and keep moving the parts
ADVICE: Performing periodic maintenance on your sewing machine is particularly important.
A well-maintained machine will likely run better and prevent annoying, time consuming, and costly problems later.
Order your maintenance kit here including needles, oil, brush, screwdriver, etc.