Didn't you read the description when you selected the SCR symbol in the component browser?
It says: "Generic SCR symbol for use with a model that you supply."
Search the web for a model for the SCR you want to use, which will be a .sub or .lib file (possibly zipped). .lib files can contain multiple models. A model in LTspice format is strongly preferred, as there are some syntax differences for behavioural sources that can cause problems if you attempt to use models intended for PSPICE or other SPICEs. However many PSPICE models are fully LTspice compatible. Download it and if its zipped extract it into the same folder as your simulation. Add a .lib or .inc statement to the schematic, naming the file containing the model. Use a .inc statement if its a .sub file or .lib if its a .lib file. Make sure the 'Value' field of the SCR symbol you have placed EXACTLY matches the name of the model inside the .lib or .sub file.