Why won’t the arbor lock engage on the Ryobi circular saw

It is very likely, that the fixating screw/disk is loose. Do not operate the saw in this state. It is dangerous.

A circular saw usually has a direct drive. I.e. the motor spindle is also the center shaft for the blade. So if your motor is spinning, but not the blade, it means the blade has lost contact to the shaft.

Check if the blade is properly centered and aligned with all locating pins (if your saw has some). Then fixate the screw using the appropriate wrenches.

Despite what you might believe, those adjustment levers on most tools are not calibrated very well. Even on my table saw I regularly use a simple plastic 90-degree triangle to ensure my blade is aligned to the table. This is part of owning any tool and it's worth a few moments to verify everything is set up properly. A more expensive tool will simply buy you more time between calibrations but all tools should be checked with regular use.