GSTR 1 (RCM)

(Because if we put it in 5% column about tax liability is generated)

Just You ll "Tick" the option of "Reverse Charge Applicable" in Your GSTR 1 return...

but it doesn't save....says to select a rate

How to you issued bills to Your customers...?
Which TAXE rate applicable for your GTA services like 5% or 12%...?

If You are under 5% slab then You ll select 5%.
After selecting the value You must select the option of "Reverse Charge Applicable"...

If You are not select the RCM option then Your liability is shown as per your feed....

service provider issued bill at taxable value.... service receiver pays due to RCM at 5%, so I selected 0% rate for the service provider

No...
If You are select the 0% value then it's consider as Sale of Goods in Your GSTIN...

So.,
RCM option is must if You are GTA...

*** (Just it's my assumption and Adviceable)
In case you are filed the GSTR 1 return as per Your view then Go to amend the Details in Your next GSTR 1 return