Expression.Error: We cannot convert the value null to type List.

Please see this article.  You want to put your query parameters (the stuff after the ?) in the Query clause, not in the RelativePath.

Chris Webb's BI Blog: Using The RelativePath And Query Options With Web.Contents() In Power Query And Power BI M Code Chris Webb's BI Blog (crossjoin.co.uk)

let
    Source = (PageNumber as number) => let
    Source = Json.Document(Web.Contents("https://api-na.myconnectwise.net/v4_6_release/apis/3.0/service/tickets",[Query=[pagesize="1000", Page=Number.ToText(PageNumber)],Headers=[Authorization="Basic <auth token here>",clientID="<client ID here>", ContentType="JSon"]]))
in
    Source
in
    Source

Regards,

Pat

Additional information that may be helpful. I am also able to pull data using that same data source with a single query but it has the 1k record limitation. The list has columns and nested records and I am able to use power bi to expand them all. 

let
    Source = Json.Document(Web.Contents("https://" & "api-na." & "myconnectwise.net/" & "v4_6_release/apis/3.0/" & "service/tickets?pageSize=1000", [Headers=[Authorization="Basic <token>", clientID="<id>"]])),
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Expanded Column1" = Table.ExpandRecordColumn(#"Converted to Table", "Column1", {"id", "summary", "recordType", "board", "status", "company", "site", "siteName", "addressLine1", "city", "stateIdentifier", "zip", "contact", "contactName", "contactPhoneNumber", "contactEmailAddress", "type", "team", "priority", "serviceLocation", "agreement", "severity", "impact", "allowAllClientsPortalView", "customerUpdatedFlag", "automaticEmailContactFlag", "automaticEmailResourceFlag", "automaticEmailCcFlag", "closedDate", "closedBy", "closedFlag", "approved", "estimatedExpenseCost", "estimatedExpenseRevenue", "estimatedProductCost", "estimatedProductRevenue", "estimatedTimeCost", "estimatedTimeRevenue", "billingMethod", "subBillingMethod", "dateResolved", "dateResplan", "dateResponded", "resolveMinutes", "resPlanMinutes", "respondMinutes", "isInSla", "resources", "hasChildTicket", "hasMergedChildTicketFlag", "billTime", "billExpenses", "billProducts", "location", "department", "mobileGuid", "sla", "slaStatus", "currency", "_info", "customFields", "country", "actualHours", "source", "poNumber", "workRole", "requiredDate", "addressLine2", "billingAmount", "automaticEmailCc", "workType", "hourlyRate", "contactPhoneExtension", "opportunity", "subType", "budgetHours", "owner", "estimatedStartDate", "duration", "parentTicketId", "mergedParentTicket", "subBillingAmount"}, {"Column1.id", "Column1.summary", "Column1.recordType", "Column1.board", "Column1.status", "Column1.company", "Column1.site", "Column1.siteName", "Column1.addressLine1", "Column1.city", "Column1.stateIdentifier", "Column1.zip", "Column1.contact", "Column1.contactName", "Column1.contactPhoneNumber", "Column1.contactEmailAddress", "Column1.type", "Column1.team", "Column1.priority", "Column1.serviceLocation", "Column1.agreement", "Column1.severity", "Column1.impact", "Column1.allowAllClientsPortalView", "Column1.customerUpdatedFlag", "Column1.automaticEmailContactFlag", "Column1.automaticEmailResourceFlag", "Column1.automaticEmailCcFlag", "Column1.closedDate", "Column1.closedBy", "Column1.closedFlag", "Column1.approved", "Column1.estimatedExpenseCost", "Column1.estimatedExpenseRevenue", "Column1.estimatedProductCost", "Column1.estimatedProductRevenue", "Column1.estimatedTimeCost", "Column1.estimatedTimeRevenue", "Column1.billingMethod", "Column1.subBillingMethod", "Column1.dateResolved", "Column1.dateResplan", "Column1.dateResponded", "Column1.resolveMinutes", "Column1.resPlanMinutes", "Column1.respondMinutes", "Column1.isInSla", "Column1.resources", "Column1.hasChildTicket", "Column1.hasMergedChildTicketFlag", "Column1.billTime", "Column1.billExpenses", "Column1.billProducts", "Column1.location", "Column1.department", "Column1.mobileGuid", "Column1.sla", "Column1.slaStatus", "Column1.currency", "Column1._info", "Column1.customFields", "Column1.country", "Column1.actualHours", "Column1.source", "Column1.poNumber", "Column1.workRole", "Column1.requiredDate", "Column1.addressLine2", "Column1.billingAmount", "Column1.automaticEmailCc", "Column1.workType", "Column1.hourlyRate", "Column1.contactPhoneExtension", "Column1.opportunity", "Column1.subType", "Column1.budgetHours", "Column1.owner", "Column1.estimatedStartDate", "Column1.duration", "Column1.parentTicketId", "Column1.mergedParentTicket", "Column1.subBillingAmount"}),
    #"Expanded Column1.board" = Table.ExpandRecordColumn(#"Expanded Column1", "Column1.board", {"id", "name"}, {"Column1.board.id", "Column1.board.name"})
in
    #"Expanded Column1.board"

PageNumber is defined as a number, so you should not have [] surrounding PageNumber.  So this: [PageNumber] should be replaced by just PageNumber.

Also, where is the definition of PageData?

Hope this helps!--Nate

Hope that helps!--Nate

By carefully examining your query, identifying the source of the null values, and applying the appropriate solutions (like replacing null with an empty list or using conditional logic), you can resolve this "cannot convert the value null to type List" error in Power Query. The key is to be proactive in handling potential null values before they cause errors.