As I said before (4 days ago)
put rethrow() in your catch sequence in order to know what are the errors (exceptions) that were catched
the 1st problem is there is no match for GRB so the m variable is not defined
you should refactor the code so the regular expression is only done once, and replace all m.match by grb variable so it is already defined (even to nogrb in case of no match).
See « modified code » of 4 days ago.
Good luck!
Still it is not renaming Tmid-T0 and Limit columns. 
using HTTP , CSV, DataFrames
function doanalysis()
dfg=nothing
for x in 34000:34146
print("\r peeking at GCN $x ")
try
url = "https://gcn.nasa.gov/circulars/$x/raw"
resp = HTTP.get(url)
status=resp.status
print(" ",status," ");
if status == 404 ; println("status=",status); continue; end
txt = String(resp.body)
grb_rexp=r"GRB ?\d{6}([A-G]|(\.\d{2}))?"
m=match(grb_rexp,txt)
grb="nogrb"
if occursin(grb_rexp,txt)
print(m.match)
grb=m.match
end
function dframe()
df.GCN=[x for i in 1:nrow(df)]
df.GRB=[grb for i in 1:nrow(df)]
if isnothing(dfg)
dfg=df
else
dfg=vcat(dfg,df,cols=:union)
end # if x is first
end # dframe
if occursin("V. Lipunov", txt)
println(" MASTER report")
he=first(findfirst(r"^Tmid"im,txt))
lr=first(findnext("\nFilter",txt,he))-1
cltxt=txt[he:lr]
df=CSV.read(IOBuffer(cltxt), DataFrame, delim='|', skipto=3)
if "Limit" in names(df); rename!(df, "Limit" => "Mag"); end
if "Filt." in names(df); rename!(df, "Filt." => "Filter"); end
if "Tmid-T0" in names(df); rename!(df, "Tmid-T0" => "Time"); end
#rename!(df, "Filt." => "Filter", "Tmid-T0" => "Time")
dframe()
elseif occursin("report on behalf of the Swift/UVOT team",txt)
println(" SWIFT/UVOT report")
hb,he=findfirst(r"^Filter"im,txt)
lr,_=findnext("\n\nThe",txt,he)
cltxt=replace(txt[hb:lr], " +/- "=>s"\t", r" +(\w)"=>s"\t\1" ,r" +(>)"=>s"\t",r"\+/?- ?"=>s"\t",">"=>s"\t")
df=CSV.read(IOBuffer(cltxt), DataFrame, delim='\t')
if "Column6" in names(df); rename!(df, :Column6 => :Mag_err); end
df.Time = (df."T_start(s)" .+ df."T_stop(s)") ./2
dframe()
end # if occursin
catch e
println("error ")
#rethrow()
end # trycatch
end # for loop
println()
if !isnothing(dfg)
CSV.write("data-all.csv",dfg)
else
@info "no dfg to write"
end # !isnothing
end # function doanalysis
doanalysis()
but below code works if i provide renaming for every column.
rename!(df,["Time","Date Time","Telescope","Coord (J2000)","Filter","Exp(s)","Mag","Comment"])