I have a tenant complaining that the AC is not cooling when the temperature get around 90 degrees but it cools in the morning and the evening when the temperature is lower and also the walls are sweating for some reason. (I don't know why this is happening) 

I went into the unit today to check the insulation in the attic and the crawl space it looks good to me and the house was cool when I went in there around 10:30am. 

How do i tell the tenant that the HVAC is working and there are no issues found? or can it be something else that i overlooked?

I think 10:30 a.m. might be too early in the day to check the unit. I would suggest trying to go in there in the early afternoon, unless the outdoor temperature was already over 90 when you checked. Maybe check the HVAC filter if it hasn't been changed recently? Also, is there anything covering the windows--blinds, etc.? That can help control the heat if the HVAC system is determined to be working effectively. 

Too many variables to be able to give a definitive answer. Is this an older home? I've got a fourplex that's 100 years old and my tenants that live upstairs complain about the heat during the day in the middle of the summer. My solution was to get black out curtains for the windows to keep the suns heat out and buy them a window unit. 

The central HVAC just can't keep up with the extreme heat down here with such a lack of proper insulation in the windows and walls. During the morning and night though, they are plenty cool. It's just during the day, and now that everyone is working from home due to COVID-19, I felt it necessary to buy the curtains and window units. 

@Abby Weinstock the system is working effectively, I have called an HVAC company and they said it was working fine. sometime when the temperature gets over 90's our the HVAC systems are not built for that high temperature and yes I have window blinds. 

@Joshua Ferrari Yes it is an older house and thanks for that advise that may work. Also do you think adding more insulation would help in the attic? 

@Sohil Shah when you say “the walls are sweating” is it in just one spot? Also, in the heat of the day when the unit has been running a while have the tenant look at the lines coming out of the outside unit and see if they have ice on them.

@John Edwards just one room where the walls are sweating and I checked the unit this morning I do not see any ice. 

@Sohil Shah when I had that same problem sweating on one wall in my home) it was actually the lines freezing due to low coolant. If you have a way to test it yourself it’s worth checking out. If not, it could be a cheap call to a HVAC specialist.

Originally posted by @Sohil Shah :

@Joshua Ferrari Yes it is an older house and thanks for that advise that may work. Also do you think adding more insulation would help in the attic? 

It could help. We insulated our attic to R-32 which is just the level of insulation in your attic 

@Joshua Ferrari - Thanks 

@Sohil Shah it could be low on refrigerant, which would only be noticed when it gets really hot. This could be a cause of something more serious in the unit. How old is the unit? It could also be that the unit can’t keep up because it’s not sized properly or just that too much afternoon sun is getting in the windows.