I worked in Ontario until August and then moved to Quebec. I did not receive an RL-1 for my work in Ontario but am being prompted to enter one anyway.

When you are entering your T4 for ON work, make sure you apply the province correctly, this may be why it is prompting you to enter a RL-1.

But if you have worked in QC then you may have a RL-1 to apply.  Please see link below for further directives if needed:

What do I do if I have a T4 slip with no matching RL-1 slip or an RL-1 slip with no matching T4? - https://turbotax.community.intuit.ca/replies/3217566