My iPad runs on iOS 11 and I can’t connect it to my iPhone XS max on iOS 16. Could the iOS be the reason it’s not connecting.

iOS16 for iPhone is perhaps very unlikely to be backward-compatible by unsupported (older) versions of iOS.

Assuming the tags appended to your post are correct (indicating an iPad Air2 with iOS11 installed), you should update your iPad. An iPad Air2 can be updated to iPadOS 15.7. Insofar as functionality allows, your iPad will be inter-workable with your iPhone XS Max with iOS16 installed.

Be aware that while an iPad may is capable of update, it is too far behind the current version to be updated directly from iPad settings. In such circumstances you will need to use a computer to perform the update. You will need access to either a Windows PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

Could be a strong possibiliity.

Your iPad model can either be upgraded to iOS 12.5.6 or iPadOS 15.7, dependimg on which iPad model you have.

Either way your iPad is waaaay too far behind in iOS/iPadOS upgrades/updates.

If you have access to a full computer, like a Mac or Windows PC, try either using the Finder on a Mac that has the newest macOS Catalina, or later macOS, installed OR with an older Mac with an older Mac OS, download and install the latest iTunes version ( or latest iTunes for Windows PC ), make sure the Mac or Windows PC is connected to the Internet, connect your iPad by the USB charge cable to the computer and let the iTunes application upgrade/update that iPad model, using the iTunes Software Update feature, to its current, maximum OS of iOS 12.5.6/iPadOS 15.6.1.

Make sure your iPad is, at least, charged to 50% or greater to attempt this.

Also, make sure your iPad has, at the very least, between 6-8 GBs of free internal data storage space for downloading and installing a major iOS/iPadOS upgrade.

Make sure your desktop/laptop computer has plenty of internal drive storage space, also.

Also, make sure to do a backup of your iPad's data, prior to upgrading/updating.

Backup to Apple iCloud and/or a full Mac or Windows PC using the same iTunes app OR, if a newer Mac running macOS Catalina, or later, making a backup using the macOS Finder app.

If NO access to a Mac or Windows computer, you will have to seek help from a good friend or family member, with a full Mac or Windows PC, to help you out

OR

Search online to locate and call contact your nearest, “local” Apple Store location to try/attempt to make an appointment to get your Apple iPad upgraded/updated to its latest iPadOS 15 version.

OR, alternatively, use the “Get Support” link, at the upper right hand corner of every Apple Support Community Webpage to attempt to schedule an appointment with your nearest Apple Store location.

Apple Store employees will be glad to help/assist you.

Best of Luck to You!