So, you said PayFlow Link in your first paragraph, but that's something completely different from Express Checkout or Payments Pro, which is what it looks like you're trying here.
If you use TENDER=C and you're getting that error it likely means you don't have Payments Pro enabled on your PayPal account. You mentioned that you do, but can you confirm that it's actually activated and setup for test mode? You should see a little table with the status of the account on the overview page when you login to the PayPal Manager site.
If you're using P to try and setup an Express Checkout flow then you would need to make sure you remove the credit card number from the request.
As a side note, the sample code you pasted here is missing a comma in the $PayPalRequestData array after the billtomiddlename parameter. I had to fix that before it would run at all.
With that fixed, I was able to successfully run your sample request with a test credit card number from the PayFlow documentation.
Array
(
[RESULT] => 0
[PNREF] => A10M6CE2A65B
[RESPMSG] => Approved
[AUTHCODE] => 000041
[AVSADDR] => Y
[AVSZIP] => N
[CVV2MATCH] => Y
[PROCAVS] => A
[PROCCVV2] => M
[TRANSTIME] => 2014-04-23 01:43:58
[BILLTOFIRSTNAME] => Tester
[BILLTOLASTNAME] => Testerson
[AMT] => 10.00
[ACCT] => 1881
[EXPDATE] => 0419
[CARDTYPE] => 0
[IAVS] => N
[RAWREQUEST] => BUTTONSOURCE[18]=AngellEYE_PHPClass&VERBOSITY[4]=HIGH&USER[6]=****&VENDOR[9]=****&PARTNER[6]=PayPal&PWD[8]=****&TENDER[1]=C&TRXTYPE[1]=S&ACCT[16]=4012888888881881&EXPDATE[4]=0419&AMT[5]=10.00&FREIGHTAMT[4]=5.00&TAXAMT[4]=2.50&COMMENT1[15]=This is a test!&COMMENT2[20]=This is only a test!&CVV2[3]=123&BILLTOPHONENUM[12]=816-555-5555&BILLTOFIRSTNAME[6]=Tester&BILLTOLASTNAME[9]=Testerson&BILLTOSTREET[13]=123 Test Ave.&BILLTOCITY[11]=Kansas City&BILLTOSTATE[2]=MO&BILLTOZIP[5]=64111&BILLTOCOUNTRY[2]=US&SHIPTOFIRSTNAME[6]=Tester&SHIPTOLASTNAME[9]=Testerson&SHIPTOSTREET[13]=123 Test Ave.&SHIPTOCITY[11]=Kansas City&SHIPTOSTATE[2]=MO&SHIPTOZIP[5]=64111&SHIPTOCOUNTRY[2]=US
[RAWRESPONSE] => RESULT=0&PNREF=A10M6CE2A65B&RESPMSG=Approved&AUTHCODE=000041&AVSADDR=Y&AVSZIP=N&CVV2MATCH=Y&PROCAVS=A&PROCCVV2=M&TRANSTIME=2014-04-23 01:43:58&BILLTOFIRSTNAME=Tester&BILLTOLASTNAME=Testerson&AMT=10.00&ACCT=1881&EXPDATE=0419&CARDTYPE=0&IAVS=N
)
I ran it with TENDER=P, too, and I got an expected error that the tender type isn't enabled on my account.
So I would double check your API credentials, and maybe even try switching it to FULL_TRANSACTIONS and see if that helps.