Lawrence thought a move from Stoke City had been agreed on transfer deadline day, but the Football League said the move was not completed before the 6pm deadline for all paperwork to be submitted.
"I was sat in the office for nearly four hours and the papers went through before 6pm so I don't know what they are disputing," he told Sky Sports.
"I will go back and see the family and then I will go straight down to Portsmouth for training on Thursday. I believe it is classed as a loan move, but it's still all the same."
Coventry City have named Martyn Pert as assistant manager to Aidy Boothroyd.
Pert, 32, had worked under Boothroyd at Watford, where he was head of conditioning.
Crystal Palace have completed the loan signing of James Vaughan on loan from Everton this week.
"It's no secret we have been looking for a striker and I think James could be the answer," manager George Burley told the Croydon Advertiser.
"He will be the final player we bring in, after this deal we will work with what we've got."
Millwall have signed striker Theo Robinson on loan from Huddersfield Town until January.