Preparing PDB file for modeling with Swiss-Model
As explained in the website the data introduced must be:


  Format must be FASTA, Clustal, plain string, or a valid UniProtKB AC


So you need to paste just the protein sequence. Without seeing the data you paste I would guess that you don't use a valid plain string.

As Jonathan Moore explains below a FASTA format is:

&gt;SEQUENCE_1 
MTEITAAMVKELRESTGAGMMDCKNALSETNGDFDKAVQLLREKGLGKAAKKADRLAAEG
LVSVKVSDDFTIAAMRPSYLSYEDLDMTFVENEYKALVAELEKENEERRRLKDPNKPEHK
IPQFASRKQLSDAILKEAEEKIKEELKAQGKPEKIWDNIIPGKMNSFIADNSQLDSKLTL
MGQFYVMDDKKTVEQVIAEKEKEFGGKIKIVEFICFEVGEGLEKKTEDFAAEVAAQL


(breaking the sequence in several lines is optional but usually done fore easy reading)