Paypal PCI Compliance
Only PayPal can tell you the reasons for an account limitation and the steps required to resolve it.

PCI stands for "Payment Card Industry", and is a standards body that Visa and MasterCard and other payment card providers are part of. They created a set of standards that merchants must meet to be compliant. PayPal does not validate, and is not responsible for, merchant's compliance when handling payment card data directly (e.g. credit card for fields on your website).  Merchants accounts who do touch credit card numbers directly (e.g. a Pro integration), and are not compliant, are likely to have their accounts deactivated.

But since you seem to be talking about a regular PayPal Express Checkout payment, which would not involve any payment card data external to PayPal's site, your client's PCI compliance is not relevant at all for this type of payment. Something else is going on with regards to it.