You can come across both in reality, but according to the rules you're supposed to take "one step back in tense" while retelling. So if a person says "can", when you retell you should say "he said he could" etc.

If you "told" your landlord this already, it has already happened. You should say, "I told my landlord that I couldn't pay the rent". However, if you want to tell exactly what was said or quote what you said, you could write the following. I told my landlord, "I can't pay the rent this month". Hope this helps

In reality both work and people would know what you mean either way. But the difference is tense. Both 1 and 2 use the word told so you have already told you land lord (past tense). Can is present tense Could is past tense/timing and that tense relates to the time you make the statement. Option 2 is correct if the statement is made at a time when you are currently required to pay rent (Present tense). Option 1 is correct if the statement is made at a time when the ability to pay rent on time has already past or is late (past tense). complicated problem. I wish may spanish was as good as your english